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The Arguments for Hydrogen Combustion Engines

26 September 2006

Bmwh21
BMW envisions the development of hydrogen combustion engines that eventually use charged, direct injection to deliver high efficiency. The current Hydrogen 7 is represented by the H2-PFI column. Click to enlarge. Source: BMW

Speaking at the California Air Resources Board Zero Emissions Vehicle (ZEV) Symposium, representatives from Sandia National Laboratories and BMW laid out the rationale and technical strategies for a focus on hydrogen-fueled combustion engines (H2ICE).

Using hydrogen with a combustion engine, according to Dr. Andy Lutz from Sandia, is a pragmatic bridge to a hydrogen economy. The technology is available today and economically viable in the short term, with fewer constraints concerning hydrogen storage compared to fuel cells. Impurities, for example, are a non-issue for a combustion engine (“You burn them right up.”).

PHydrogen engines have demonstrated efficiencies (BTE) in excess of today’s gasoline engines, NOx is the only regulated tailpipe pollutant resulting from hydrogen combustion, and carbon dioxide is a non-issue, at least in terms of the driving (Tank-to-Wheels) component of the lifecycle.

Although current efforts by Ford and BMW reflect early stage development, both BMW and Sandia outlined technology approaches for subsequent generations of H2ICE systems that could deliver significant improvements in fuel economy and emissions reduction, while delivering additional power.

Bmwh23
BMW’s projected product pathway. Click to enlarge. Source: BMW.

Dr. Edgar Berger from BMW, in particular, described a future generation H2ICE 4-cylinder engine that could deliver more than 140 kW (188 hp) of power with fuel consumption of 1.4 to 1.6 kg H2/100 km.

One can reach, in fact, 1kg/100km H2—but the price is to reduce vehicle properties and customer benefits.

—Edgar Berger

In terms of its basic combustion properties, hydrogen offers certain benefits and certain challenges compared to gasoline. (See table below.)

Sandiah2ice1
Combustion properties of gasoline, CNG and hydrogen. Favorable hydrogen properties are tagged in blue; unfavorable in red. Click to enlarge. Source: Sandia National Laboratories

Its wide flammability range (Φ) supports a much leaner burn mixture—a factor that is important for emissions management strategies. The much higher laminar flame velocity produces stable flames under more extenuating circumstances, and, combined with the higher autoignition temperature, creates a higher research octane number that supports higher compression.

On the downside, hydrogen has a high stoichiometric volume fraction, which affects how much charge passes through the engine in a given displacement, and in turn affects the power of the engine.

It also has a lower minimum ignition energy and hence has a tendency to pre-ignite.

The researchers at Sandia have identified five possible approaches to dealing with the challenges posed by hydrogen combustion.

  1. Continuous ultra-lean (Φ<0.45) operation with improved power densities. This, combined with turbo- or supercharging is the approach Ford is taking with its H2ICE Focus passenger car and E450 shuttle bus. For also has a H2ICE-hybrid research vehicle—the H2RV— that combines a 2.3-liter combustion engine with a 30 hp electric motor. All vehicles deliver SULEV emissions or better.

  2. Operate at stoichiometric conditions (Φ=1) with aftertreatment. Possible routes within this strategy include the use of liquid fueling to prevent preignition if the fuel can be kept cold to the point of injection; direct injection, and the use of Exhaust Gas recirculation.

  3. A multi-mode strategy. This is the approach BMW is taking with its Hydrogen 7, running ultra-lean under partial load to minimize engine-out NOx, and at stoichiometric condition under full load, coupled with the use of a three-way catalyst to handle the resulting NOx. (Earlier post.)

  4. Another variation of the multi-mode strategy uses ultra-lean mixes at low load, pressure boost in the medium range, and then lean NOx traps at high load. Ford is looking into this for the H2ICE Focus.

  5. Sandiah2ice2
    Click to enlarge. Source: Sandia National Laboratories
    Mixture stratification with direct injection. This approach would use a stratified and extremely lean mix at idle. At low-load, it would move to an ultra-lean homogeneous mixture. As load increases, the system would start using stratification with direct injection, and then rely on the lean NOx trap at high load. Sandia concludes that such an approach could theoretically deliver BTE of greater than 45%, with emissions significantly below SULEV.

It’s complicated, but with electronic controls there are a variety of things that can be done.

—Andy Lutz

For its part, BMW outlined an ambitious development plan that it intends to result in mono-fuel hydrogen engines with greatly improved efficiency and reduced fuel consumption that it can apply across its entire model range, from luxury to compact.

Bmwh22
Advanced energy management. Click to enlarge. Source: BMW.

Mirroring some of the Sandia work, BMW is ultimately looking toward a charged, direct-injection engine as a future generation platform. Berger also described a hybrid architecture that would combine a small fuel cell with the hydrogen combustion engine to augment electric power for vehicle subsystems and traction power.

A key enabler for this strategy is having sufficient hydrogen on-board to fuel the engine. BMW has already opted for liquid hydrogen storage, with its higher volumetric and gravimetric densities than offered by compressed hydrogen.

Bmwh24
Volume and weight of different methods of storing 10 kg of hydrogen, which is equivalent in energy to 38 liters of gasoline. Click to enlarge. Source: BMW.

However, BMW believes that it needs to have 10kg on board hydrogen to met its performance and customer satisfaction objectives. Currently, the Hydrogen 7 stores 8 kg in a 150-liter container.

Accordingly, BMW has work underway to expand the storage density of its liquid hydrogen storage, to decrease the boil-off loss, and to increase the loss-free dormancy time.

Furthermore, for its 5 Series size cars, BMW is developing a shaped storage tank it calls the “double bubble”—a single-tank system providing central storage running down the midline of the car in the tunnel.

Bmwh25
BMW’s hydrogen storage roadmap. Click to enlarge. Source: BMW.

Ultimately, it sees using liquid hydrogen in the larger classes (luxury and executive) with 7.5 to 10 kg in a given total package of 250-300 liters. For small to medium-class vehicles, BMW is looking at compressed hydrogen, and possibly some activity with cryo-compressed hydrogen.

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September 26, 2006 in Engines, Hydrogen | Permalink | Comments (52) | TrackBack (0)

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> Using hydrogen with a combustion engine, according to Dr. Andy Lutz from Sandia, is a pragmatic bridge to a hydrogen economy.

Yeah - a bridge to nowhere.

BMW's system could be an excellent starting point for short-hop passenger and cargo aviation applications. For vehicles, LH2 makes no economic and little if any ecological sense.

All that talk about engines, and nothing about the big issue:  how to get the hydrogen.

What if you want to leave your car for a month? Do you have to vent the liquid hydrogen tank? There is no way to eliminate boil off of LH2. Finally it takes something like 40% of the available energy to liquify hydrogen which is much worse than the 20% or so needed to compress it to 10000psi.

The recently announced hydrogen BMW 7 series loses (boils off) its fuel in 9 days! Lose one-ninth of your fuel every day; what a deal!! Not ready for prime time.

If you're burning the fuel in an IC engine, you might as well stick with hydrocarbon fuels and balance emissions by extracting CO2 from the air or ocean surface waters. The fuel tank would be much smaller (even if you use CNG).

The planet cannot afford to burn extra gas and coal in power stations to make hydrogen to burn in engines. CO2 on a well to wheel basis is awful.......just about the worst fuel possible.

BMW should burn CNG in their cars, the filling stations exist in Germany, no range issue, can even use bio-methane from landfill to run BMWs...truly sustaianable performance. This is inevitiable.

With an H2 ICE, you need just about as much energy as you would for a gasoline ICE as the efficiency of the H2 ICE is not much better. With a H2 FCV, tank-to-wheels efficiency improves, and you need less onboard storage. This is yet another reason why H2 ICEs are not going to be a very practical bet (despite what BMW says), the main reason being that the well-to-wheels efficiency and emissions are horrible.

Energy storage density is obviously a problem with PHEVs and EVs too, (it would be cool to see a similar graphic for that), but you get 3-5x the tank-to-wheels efficiency, so you need 1/3-1/5 the on-board energy storage, which is not the case with H2 ICEs and even FCVs aren't anywhere near as close (a little more than 2x the efficiency if I remember correctly). Plus you get boil-off if you are using LH2 so you lose fuel if your car sits parked for a while (which I can't imagine consumers would like).

PHEVs are a much better short-term bridge to an electrified vehicle fleet. H2 ICEs are a bridge to more or less nowhere.

Ah, the argument for H2-ICE is very strong. As I've suspected, the efficiency for H2-ICE can approach 45% using the mixture stratification with direct injection. Of course, all ICE's are most efficient at only at a narrow range between 50%-70% of maximum throttle, meaning that an ICE-electric hybrid will be needed to keep the engine running only in its most efficient range of 45% BTE for most of the time. The gasoline Prius is already calculated by Toyota to have a tank to wheel efficiency of 37%, so 45% efficiency for H2-ICE-electric hybrid is a realistic expectation. In comparison to the Honda's upcoming fuelcell FCX with announced 60% efficiency, this is not so bad, given the lower cost and the higher durability of H2-ICE's in comparison to PEM fuelcells.

To all H2 doubters,

The first thing to remember is that H2 can be produced very easily and very efficiently from almost any type of combustible feedstocks. Meaning that a gasoline station in remote areas can be supplied with crude oil, or natural gas, waste biomass etc. and can produce H2 on the spot. A car running on H2 can vicarously use any type of combustible fuels, and not dependent ONLY on one type, such as gasoline car on gasoline with appropriate octane rating with added ethanol or other kind of additives, or diesel depended on low sulfur fuel with appropriate cetane rating. Gas stations in large cities can have H2 piped-in from a local central H2 reforming plant, with optional CO2 sequestration means.

Given the inefficiency and great expense of H2 production from electrolysis of water at room temp, do not expect this to be the source of H2 for mass consumption. There are other methods of H2 production from renewable energy that can rival the BEV or PHEV in term of total source-to-wheel efficiency. Please look at this link http://www.greencarcongress.com/2006/09/quantums_compre.html
for my last posting that discuss highly-efficient way to generate H2 that can rival the BEV in term of total efficiency.

Rafael,

I think that BMW plans to use compressed H2 for smaller or lower-cost automobiles.
But, for the high-end autos with heavy weights due to over-abundance of luxurious items, compressed H2 do not offer sufficient volumetric efficiency as LH2 for sufficient range, ergo the use of LH2. The efficiency of LH2 sucks in comparison to compressed H2, but the rich surely can afford it. Those who are wealthy enough to purchase BMW's costing over 100,000 USD do not have to worry about fuel efficiency, only about how far between fill-up, as their time is worth a lot of money. (eg. lawyers at >$150 hourly fee)

LH2 is excellent for commercial aviation jets, and the value of LH2 is not just limited to short hops. The longer the range, the more energy-saving when using LH2. Those who wonders why this is so can look for my previous posting on the same subject.

...continues from above,
This article left out the most important advantage of H2-ICE-electric hybrid over that of FCV: the ability to combust methane-H2 mixture at any ratio. Methane allows over 3 times the volumetric energy density of H2. Thus, the compressed H2 tank needs not be overly large, but just sufficient for a range of ~120-150 miles. If a range of 360-450 miles is desired, just fill up the tank with a mixture of up to 90% methane and 10% H2 (to improve ignitability), and this is a much more cost-effective solution over that of LH2. Of course, in the future, H2 is expected to be produced cheaper and more energy-efficient than methane from renewable energy sources, so most people will probably choose H2 for daily commute. A fill up every 4-5 days with H2 for daily commute is probably Okay, as it will give one time to grab a cup of coffee, a donut or time to glance at the newstand at a local friendly 7-11 gas station. Those high-dollar doctors or lawyers probably will choose to fill-up with methane instead, and pay a price premium. But, hey, this a free capitalistic country. Different strokes for different folks! Those BEV enthusiasts may choose to miss out on the brief weekly social gatherings at 7-11 gas stations altogether and plug in their vehicles at home instead.

H2 can only be made efficiently from chemical fuels (mostly hydrocarbons, alcohols and the like).  Electricity can be made from all of the above, and much more.  Further, the efficiency of gasification to H2 is around 80%, yielding 36% best-case throughput in a 45%-efficient engine; current batteries can hit 90+% efficiency.  Even if you burn natural gas in a CC turbine to make juice (60%), you can get 54% throughput - 50% better than hydrogen.

Energy security is the other side of the coin.  The fewer the options for the energy supply, the less secure you are.  Hydrogen is inherently less secure than electricity.

Eng-Poet,
You're getting very close. CC Turbine has an efficiency of 60%, however, transmission to the home socket is only 90% efficient, leaving 54% efficiency from power plant to home socket. BEV is generally accepted as having a grid to wheel efficiency of ~70%, considering all the losses in the charger, battery, power inverter, resistance in the motor, friction in the drive train, etc...So, multiplying 54% by 70% will leave you with 37.8% total efficiency for BEV, which is not too far from a calculated number of 36% overall efficiency for an optimized H2-ICE-electric hybrid vehicle. Now, if your electricity comes from coal in conventional coal power plants having ~40% efficiency, as in 50% of electricity production in the USA, and multiply by 90% efficiency from power plant to home socket, and multiply by 70% efficiency of BEV, you'll have only 25% efficiency for BEV from coal to electricity. Whereas, if you gasify this coal and produce H2, you'll have higher efficiency for H2-car.

Your hydrogen car efficiencies don't include drivetrain losses, so you should omit them from the EV example too.  You also assume that the H2-combustion car will always be running at its best efficiency - it can't, while the EV mostly will.

Eng-Poet,
The Prius has a tank to wheel efficiency of 37% as posted by Toyota's site. This includes all drive train losses. Due to the higher efficiency of H2 combustion, including higher compression ratio, isochoric combustion process, and ultra-lean burn regime (thanks to high ignitability of H2 at very low concentration) in which less heat rejection into coolant and more heat available to do work, an increase from 37% tank to wheel to 45% tank-to-wheel would be achievable. A full hybrid drive train would ensure that the engine is used only in the most efficient power range. EV's don't run at its highest efficiency in hard acceleration, either, due to higher resistive losses in all circuitries including motor windings, power inverter and batteries. If you race R/C electric cars or fly R/C electric planes, you would realize this. The higher the power setting, more electrical energy will be wasted to heat production.

All those advantages are reversed if you don't start with a fossil fuel.  That's my complaint about hydrogen:  it looks like a scheme to lock in the interests of the fossil-fuel corporations for the next century, and the hell with the environment.

Engineer-Poet is certainly on the proper side of the equation. We all know that it is much easier (and much cleaner) to make, transport and use electricity (in road vehicles) than Hydrogen.

Our electricity is 98%+ from Hydro and will be a combination of wind + Hydro for decades to come. We could supply enough clean electrical energy for most PHEVs and EVs in eastern USA for an extended period.

Our support and efforts should go for the development of on-board high efficiency Energy Storage Devices required for PHEVs + improved PHEVs + EVs and the progressive elimination of noisy polluting ICE vehicles.

Of course, more clean electrical energy should be produced from Hydro-Wind-Sun-Waves (and improved nulear)to meet the new demand created by a few (many) million PHEVs and EVs. It can be done. Most industrial countries have the required potential ounce the demand is created.

Roger Pham,
From what I've gathered the 37% is maximal engine bte efficiency, and is generally not the efficiency the engine is usually operating at, which is more like 20-25% EPA highway, and 25-30% assuming the usual 70mph regime most Americans use. Peak efficiency is not average efficiency, which is what should really be assessed, and why in the ICE realm, diesels tend to get twice the mileage (btu corrected) that gassers do.

High BTE (>40%) over a significant portion of the rpm/pressure map has been shown with SI PFI methanol fueled engines, so why bother with a fuel that's so much of a pain to produce/store when we have engines right now that can exhibit these high thermal efficiencies?

Please, Mr. Please...(don't play me 17...) if u r old enuff to remember the song!

A typical modern gasoline car is accepted as having a overall BTE of 20% combined driving tank to wheel. (older cars with carburetor were accepted as having BTE of 15% combined driving). A gasoline car the size of the Prius should have a combined fuel efficiency of ~30mpg EPA. The Prius has a combined EPA mpg of 55mpg. Take 20% divide by 30 and multiply by 55 will get ya 37%. The 37% for the Prius reflects combined driving, NOT peak engine efficiency. A full hybrid drive train is designed to let the engine run as much at its peak efficiency as possible. For that reason, full hybrids engines don't idle, and don't run at low car speed, either. The electric motor does all the low-speed traveling. It only comes on strong at near its peak BTE to recharge the battery, or to pull the car at cruise. The engine is turned off during coasting also. The engine is down-sized significantly so that at cruise, it runs close to its peak BTE. The Prius has a 1.5 liter engine, but it takes in air only 1.2 liter per intake stroke due to its over-expansion power stroke, so it's comparable to a 1.2 liter engine powering an almost 3000-lb car.

Mr. Pham, I think you are refering to the maximal BTE of these engines. As you can see from this consumption/emissions model of the 1NZ-FXE, calibrated from test data, the peak BTE of the engine is roughly 37%.

http://www-personal.engin.umd.umich.edu/~chrismi/downloads/HEVModel/FC_PRIUS_JPN.m

The average efficiency during the EPA highway cycle is something like 20-25% because the car only requires ~15hp of the ~70hp the engine can produce. The 37% efficiency is the peak efficiency of that engine, but the engine is usually not operating at that level of efficiency, and drivers will not see it unless they engage in rather extreme driving habits. Or speed. ;)

Run the numbers compared to the EPA highway mileage yourself. The Prius glider needs, at most, ~8kwh to go 50 miles in an hour using the EPA highway schedule, but it uses roughly 33.6kwh, i.e. a gallon of gas. Which translates to ~24% BTE.

If Toyota had used a smaller displacement version, then perhaps more drivers could see the 36% peak efficiency at ~50mph, but Toyota knew that most drivers travel well over 70mph, so they optimized engine efficiency for much higher speeds instead of the EPA highway testing average of 48.3mph.

Mr. Please, thanks for your seriousness in presenting data to support your assertion. Your approach is truly scientific. It is, then, in this scientific spirit that I would like to point out a few things in those set of data:
Below are the data from the link that you've provided.

(g/s), fuel use map indexed vertically by fc_map_spd and
% horizontally by fc_map_trq
% fuel use from Feng An's model calibrated with actual data for Prius_jpn (Atkinson cycle) engine
fc_fuel_map = [
0.1513 0.1984 0.2455 0.2925 0.3396 0.3867 0.4338 0.4808 0.5279 0.5279 0.5279 0.5279
0.1834 0.2423 0.3011 0.3599 0.4188 0.4776 0.5365 0.5953 0.6541 0.6689 0.6689 0.6689
0.2145 0.2851 0.3557 0.4263 0.4969 0.5675 0.6381 0.7087 0.7793 0.8146 0.8146 0.8146
0.2451 0.3274 0.4098 0.4922 0.5746 0.6570 0.7393 0.8217 0.9041 0.9659 0.9659 0.9659
0.2759 0.3700 0.4642 0.5583 0.6525 0.7466 0.8408 0.9349 1.0291 1.1232 1.1232 1.1232
0.3076 0.4135 0.5194 0.6253 0.7312 0.8371 0.9430 1.0490 1.1549 1.2608 1.2873 1.2873
0.3407 0.4584 0.5761 0.6937 0.8114 0.9291 1.0468 1.1645 1.2822 1.3998 1.4587 1.4587
0.3773 0.5068 0.6362 0.7657 0.8951 1.0246 1.1540 1.2835 1.4129 1.5424 1.6395 1.6395
0.4200 0.5612 0.7024 0.8436 0.9849 1.1261 1.2673 1.4085 1.5497 1.6910 1.8322 1.8322
0.4701 0.6231 0.7761 0.9290 1.0820 1.2350 1.3880 1.5410 1.6940 1.8470 1.9999 2.0382
0.5290 0.6938 0.8585 1.0233 1.1880 1.3528 1.5175 1.6823 1.8470 2.0118 2.1766 2.2589
0.6789 0.8672 1.0555 1.2438 1.4321 1.6204 1.8087 1.9970 2.1852 2.3735 2.5618 2.7501 ];

[T,w]=meshgrid(fc_map_trq, fc_map_spd);
fc_map_kW=T.*w/1000;
fc_fuel_map_gpkWh=fc_fuel_map./fc_map_kW*3600;

%Eff
%0.13 0.20 0.25 0.28 0.30 0.31 0.33 0.34 0.34 0.34 0.34 0.34
%0.14 0.21 0.25 0.28 0.30 0.32 0.33 0.34 0.35 0.35 0.35 0.35
%0.14 0.21 0.26 0.28 0.31 0.32 0.33 0.34 0.35 0.35 0.35 0.35
%0.14 0.22 0.26 0.29 0.31 0.32 0.34 0.34 0.35 0.36 0.36 0.36
%0.15 0.22 0.26 0.29 0.31 0.33 0.34 0.35 0.35 0.36 0.36 0.36
%0.15 0.22 0.26 0.29 0.31 0.33 0.34 0.35 0.35 0.36 0.36 0.36
%0.15 0.22 0.26 0.29 0.31 0.33 0.34 0.35 0.35 0.36 0.36 0.36
%0.15 0.22 0.26 0.29 0.31 0.33 0.34 0.35 0.35 0.36 0.36 0.36
%0.14 0.22 0.26 0.29 0.31 0.32 0.34 0.34 0.35 0.36 0.36 0.36
%0.14 0.21 0.25 0.28 0.30 0.32 0.33 0.34 0.35 0.36 0.36 0.36
%0.13 0.20 0.25 0.28 0.30 0.31 0.33 0.34 0.34 0.35 0.36 0.36
%0.12 0.19 0.23 0.26 0.28 0.30 0.31 0.32 0.33 0.34 0.35 0.35
%Min bsfc = 224
%Max Eff = 0.36

Please refer to the link itself for the actual appearance of the matrices, because when I copied them over, the 12 x 12 rows and colums arrangement are lost.

1) The data are obtained in 1999, from earlier model of Prius for which Toyota has listed the tank to wheel as being 32%, in consideration of the losses in the drive train. The 37% efficiency from tank to wheel that I quoted is referred to the 2004 Prius or later, from which substantial improvement in efficiency was realized.

2) Noted that maximum BTE of 36% is obtained quite early on at relatively low rpm and low fuel consumption of only .9 g/s but at high volumetric efficiency (near wide-open throttle). Note that the maximum fuel consumption of 2.7 g/s at maximum rpm is associated with a BTE of 35%, not bad, but note that this is three times the horsepower of the maximum BTE at lower rpm. Now, look at the 9th column second row for fuel consumption, and the value 0.6541 g/s is shown. Look in the BTE matrix at 9th column second row, and this corresponds with 35% BTE. Divide 0.6541 by 2.7 and you'll get 0.24. So, let's say that maximum power is 75hp and 24% of max hp is 18hp, you'll still get a BTE of 35%, NOT BAD, ain't it. 18hp is the right hp level required for cruising at ~60mph or higher. At 50 mph cruise, the hp requirement is lower, let's say 14-16, and now, look at column 8 second row, a BTE of 34% is still available at 1/5 of maximum power (16hp). Please note that even though BTE of engine is lower at 50mph cruise, but aerodynamic drag and drive train friction will be a lot less, so the well-to-wheel data won't change to below the 32% number that Toyota has quoted.

3) The 2004 Prius should be more efficient, I suspect engine BTE is > 40%, allowing a well-to-wheel efficiency of 37%. So, by the same logic, cruising at 50 mph with lower engine BTE but lower friction loss, OR cruising at 60 mph or above, with higher engine BTE but also higher drive train friction, and your well-to-wheel number will not be lower than the 37% that I've quoted.

Your number of the Prius' engine BTE of 25% at highway cruise is NOT SUPPORTED by the EXPERIMENTAL DATA that you've provided. But, thanks for the wealth of data that you've referred me to. You have the approach of a good scientist.

Ah, I gotcha. Well, it seems to specifically depend on what rpm the Prius is at during the EPA cycle. What's the minimum rpm the Prius can cruise at from ~50-55mph? If the car can stand say, idling down the freeway, <800@50-55mph, that'd be outstanding because pumping losses would be minimized as load is maximized. Unfortunately, I don't think this is the case, because of the trade off wrt drivability so to speak.

For example, here's a tidbit about the average highway engine speed I found Googling.

"It can be done in a minimally instrumented Prius by watching a tach --keeping RPM between 1400 and 2200 at highway speeds, and more like 1200 and 2000 at lower speeds. If you have a vacuum gauge, the observed range starts as vacuum drops to 5 in-Hg or lower as demand increases, and tops out before it sinks lower to 2 or 3 while crossing 2300 RPM."

So if the engine speed is between 1.4k and 2.2k rpm. On the high side, that's half of the max engine speed, and using your example as a reference, would correspond to ~31% BTE. Now, the thing is, bouncing around on the freeway would probably result in less than ideal engine efficiency because of the changes in load needed to accelerate/decelerate according to the EPA highway schedule, which explains why the EPA highway mileage is only 51mpg, but some can get ~60+mpg with the cruise control at ~50mph, and others can get upwards of 70mpg by pulsing up to some target speed, and gliding back down to another, with the same approximate average speed.

More importantly, it begs the question, what is the average engine speed compared to some average speed, and how much can it be optimized? In terms of peak BTE, from what I've gathered, there's not much that can be done, so I'm guessing all improvement between first and second gen came from improved engine loading wrt transmission/engine speed. For instance, the 1VZ-FE has a minimal BSFC of ~237g/kwh, which corresponds to ~34% BTE, and this was an engine introduced in 1993 iirc, so the greatest improvements in gasoline engine efficiency have come during partial load, not from an increase in peak BTE. I don't think peak BTE has changed much between the 1NZ-FXE used in the first and second gen Prius', as opposed to maximized engine load from reworking the whole system, as well as a reduction in the CdA.

I'd love to see a 1L turbocharged Miller cycle Prius with ~30-40 miles of plug-in range, since that would minimize pumping losses even further, resulting in 30+% efficiencies at even lower speeds. While allowing for the same peak power output with the potential of even greater peak BTE. For instance I've "heard" that VW's TDI line is capable of 40+% BTE thanks to turbocharging, but in any event, I still don't think the Prius is seeing 35% BTE at ~50mph. I'd love for you to dig up some stuff on the CVT since I haven't been able to find much concrete info, and on that note... The ball's in your court. ;)

One more thing, if the Prius has an overall tank to wheel efficiency of 37%, why does it only get ~55mpg during the EPA highway tests? Shouldn't it get something like 80+mpg?

Ah, Mr. Please, thanks for the additional bit of data. The "1200-2000 rpm for cruising at lower speeds" seems about right. There is no actual rpm data posted on the U of Michigan Prius engine testing website, but it's seems that the second row data would correlate with engine rpm somewhere at 1200 to 1500 rpm. The second row of the matrices correspond with highest BTE at lowest power output at the lowest possible rpm.

The "trade off WRT driveability" that you are referring to would be valid in a 4-speed transmission, wherein you don't want to lug the engine to the max while cruising, because you won't have any available torque reserve at that rpm if you wanna accelerate even a little bit. Your throttle is already opened pretty wide, such that pressing further down on the gas will not cause much of any further acceleration. Your transmission will have to immediately downshift to 3rd gear with any kind of extra torque demand, but, doing so continously will wear out your transmission clutches and shifting mechanism. So, in a conventional fixed-gear-ratios transmission, you must cruise at ~above 2000 rpm and run the engine at ~35% of maximum volumetric efficiency in order to have the torque and power reserve for acceleration without downshifting. This will lower your BTE quite a bit, and you're right, to below 30%.
HOWEVER, this is a true CVT in the Prius, in which they can afford to have the engine lugged down as much as possible to bring out the maximum BTE. If acceleration is required, the simulated gear ratio will be immediately downshifted with infinitely-variable ratio WITHOUT any clutches or valves hence nothing to wear out, and the engine will immediately speeds up to deliver the higher torque required for acceleration. The engine can pretty much maintain its maximum BTE throughout its usage thanks to the electrical CVT and hybrid drive train.
How does Toyota HSD does it? The impedance on the circuitry of the starter/generator (SG) of the HSD is assigned the task of controlling the torque demand from the engine. More load on the SG will lug down the engine to simulate high gear, and less torque on the SG will unload the engine to simulate lower gear. You step down on the gas, and the SG will ease the torque load on the engine, allowing the engine to rev up and spinning the SG much faster than before, thereby sending a higher current to the traction motor thus multiplying the torque at the traction wheels that are driven partially by the electric motor and partially by direct torque from the engine in a power split planetary gear set that is the central part of the HSD.

You doubted that the BTE can't be improved in the Prius engines from the first generation to the second? You've just provide the info that the BTE of the 1VZ-FE engine 1993 model was ~34% and that was improved by the 1NZ-FXE to 36% circa 1997-1998. From 1998 to 2004 is a long stretch of time, such that improvement from 36% to ~40% BTE is not unreasonable. Toyota website has listed the improvement of the 1st gen Prius efficiency of 32% to the 37% efficiency of the 2nd gen Prius, and since the peak BTE of the 1st gen Prius engine is only 36%, it must follow that the BTE of the 2nd gen Prius engine has improved enough to bring the tank-to-wheel efficiency of the 2nd gen Prius to 37%, along with improvement of the drive train efficiency. Even with 50% efficiency improvement of the drive train, the resultant overall efficiency improvement tank-to-wheel is only but 1-2%, since drive train loss is but a few percents of the entire losses.

Mr. Please, the following is a link to Toyota's website that lists the Prius tank-to-wheel efficiency, in case you have doubt :(
http://www.toyota.co.jp/en/tech/environment/fchv/fchv12.html

For someone who's in the scientific spirit, you sure are pushing the data a bit hard in the direction you want to go. For instance, claiming that

"The "1200-2000 rpm for cruising at lower speeds" seems about right."

with no verification isn't very scientific. I asked around and was pointed to this Prius driving simulator.

http://www.wind.sannet.ne.jp/m_matsu/prius/ThsSimu/index_i18n.html?Language=en?Country=US

After fooling around with it, you'll notice that the average rpm during the EPA highway cycle (average speed of 48mph) is something like 1600-1700rpm. Definitely not 1200-1500rpm.

As for why the car must be at that rpm for drivability, if the vehicle were say, idling down the freeway at 1000rpm, then because it uses a CVT transmission, instead of a delay when the car would normally be shifting between gears, there is a delay because the ECU must inject more fuel in order for the engine to rev to roughly 4000rpm. The lower the transmission is while cruising on the freeway, the longer that gap will be between when the car is cruising, and when it is making peak power. Anecdotally, Toyota decided that having the car running at ~1700rpm@48mph was suitable for agreeable acceleration on the highway. Even though ideally it could be running at something like 1000rpm.

Now, in terms of the energy required for the car to go down the freeway at an average of 48mph, if what the Toyota website shows is true, then, since there are 33.6kwh in a gallon of gasoline, in order to be 37% efficient tank to wheels, the Prius glider itself would need .37(33.6kwh)=12.43kwh to go 51miles@48mph, according to the EPA highway mileage. So, the vehicle itself needs roughly 12.43kwh/51miles=243wh/mile at 48mph. Which, without context seems o.k.

With context, it's a bit out there. Take, for example, an S-10 converted to EV.

http://avt.inel.gov/pdf/fsev/eva/s10.pdf

At 45mph it uses 214wh/mile. And since the controller and motor aren't perfectly efficient, probably something like 80% efficient, the S-10 glider probably only needs ~.8(225wh/mile)=180wh/mile at 48mph. So, if the Prius is indeed running at 37% efficiency, then it needs 50wh/mile more than a two ton pickup with the aerodynamics of a brick, er, pickup.

If this is the case, something's fishy. Otoh, if we look at the Prius' stats in terms of Crr, weight, and CdA, then we find something interesting, and a bit more plausible.
Google returns that the Prius has a Cd=.26, and reference/frontal area=2.16m^2. It comes with LRR tires, which on the high end, supposedly have a Crr=.1, and the low end .06, so .08 seems reasonable. It weighs ~3000lbs with the driver, and the speed we're at is 48mph for the EPA highway test. Looking at the sum of the rolling and fluid friction, we come up with an instantaneous force
of (13350N).008+.5(1.225kg/m^3)(.26)(2.16m^2)(21.5m/s)^2=
107N+159N=266N, and the energy needed at that speed to go 51 miles is 266N(21.5m/s)=5.72kwh. or 5.72kwh/51miles=112wh/mile, or assuming the 8kwh figure I mentioned earlier (aircon, lights, and stereo on at full blast, rough road, etc...) 150wh/mile. Now that seems reasonable, that a lighter, more aerodynamic vehicle with LRR tires needs ~110-150wh/mile compared to a 4000lb pickup, which needs ~180wh/mile at the same speed.

Unfortunately it also means that the Prius is only operating at (112-150wh/mile)/(634wh/mile)=18-24% tank to wheels efficiency during the EPA highway cycle. Not 37%. If I were a speculative person, which I am, I would guess that someone in PR picked up on the peak efficiency of the 1NZ-FXE and ran with it as the tank to wheel efficiency. But who knows, maybe the Prius does in fact require more energy per mile than a two ton pickup truck. That would certainly be a feat of engineering...

;)

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