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Toyota implements Atkinson cycle in non-hybrid ESTEC engine; up to 38% thermal efficiency and improved fuel economy

20 June 2014

   
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Performance of the new ESTEC engine (1NR-FKE) compared to the current 1NR-FE engine applied in the Yaris. Yamada et al. Click to enlarge.

Engineers at Toyota have developed an approach to applying the Atkinson cycle—used in the engines in Toyota hybrids since 1997—for engines in conventional, non-hybrid vehicles. The Atkinson cycle with a high compression ratio is a common approach that hybrid vehicle engines use to enhance thermal efficiency. However, the drawback of the high compression ratio is a reduction of engine torque; in a hybrid, the motor torque compensates for this reduction in engine torque. Thermal efficiency at low load areas is relatively more important with conventional engines than with hybrid engines and how to overcome these issues is significantly important with conventional engines, the Toyota developers observed.

The result of their work, presented in papers this spring at the SAE 2014 World Congress, the Vienna Motor Symposium, and the JSAE Annual Congress, is the new 1.3-liter ESTEC (Economy with Superior Thermal Efficient Combustion) in-line 4-cylinder Gasoline Engine (1NR-FKE). The ESTEC 1NR-FKE achieves 73 kW of output (3 kW higher than the 1NR-FE used in Toyota A and B segment vehicles such as Yaris, iQ, etc.) with thermal efficiency of up to 38%—equivalent to the engines in hybrids. Further, at low loads, the ESTEC engine improves fuel consumption by 11% in the JC08 mode.

Improving the vehicle fuel economy is a must due to the climate change and energy issues. Enhancing the engine’s thermal efficiency contributes to lowering the vehicle fuel economy, manufactures, suppliers, and most research institutions are making strong efforts to improve this. … Engine thermal efficiency for conventional vehicles is now around 36% [and] the engine thermal efficiency for HVs is raised to more than 38%. In regards to the engine technologies for HVs, the Atkinson cycle, cooled EGR, electric water pump and low friction technology play an important role for enhancing the engine thermal efficiency.

In the future, it is expected that technologies used for HVs will be applied to the conventional vehicles and the engine thermal efficiency of both engines will be raised to more than 40%. Since these technologies also improve the engine thermal efficiency at low load areas, it is being considered whether these technologies will overcome the weakness of NA [naturally aspirated] gasoline engines. As for the future direction, large amounts of cooled EGR and lean burn technologies are essential to achieve over 40% engine thermal efficiency. This means that the role of combustion gets more important for the engine development. In addition, the improvement of fundamental technologies such as low friction and valve train system need to be looked at.

The engines with technologies contributing to lowering the vehicle fuel economy are described as ESTEC (Economy with Superior Thermal Efficient Combustion) engines from now on.

—Yamada et al.

   
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Trend of engine efficiency. Toyota indicated the placement of the new ESTEC engine. Yamada et al. Click to enlarge.   Future directions in engine efficiency. Yamada et al. Click to enlarge.

Basic components of the ESTEC. The ESTEC engine combines the Atkinson combustion cycle with a geometric compression ratio of 13.5:1 and water-cooled EGR. (The conventional 1NR-FE has a compression ratio of 11.5:1 and internal EGR.) An electrical Variable Valve Timing system (VVT-iE) is a key element for implementing the Atkinson cycle. VVT-iE enables quick and precise control of the phasing of the intake valves, and avoids restrictions that might result from oil temperature and pressure variations in cold starts.

The efficient EGR cooler uses a quick response EGR valve. Additionally, the intake manifold, EGR cooler and EGR valve are directly connected to reduce the chance of condensation from the cooler.

An intake port with a high tumble and flow volume enables rapid combustion and helps reduce knock. To meet both performance and fuel consumption requirements, a 4-2-1 pipe exhaust manifold has been designed to reduce residual exhaust gas in combustion.

Friction reduction technologies also play a role.

Performance recovery. Increasing the CR to 13.5:1 dropped torque from 104 N·m to 96 N·m. To recover the torque, Toyota used a combination of a modified exhaust manifold shape to reduce residual gas to compensate for the increase of gas temperatures with the CR increase; optimized the temperature on the cylinder surface through the use of new water jackets; and optimized injection timing. The combination of these (with the modified exhaust manifold producing the bulk of the recovery), brought torque up to 105 N·m.

At low load, cooled-EGR results in excessive torque fluctuation. To address this, at low load, Exhaust VVT is advanced and internal EGR is used. At the middle to high load areas, Exhaust VVT is retarded and the EGR valve step is advanced.

While cooling works as a countermeasure against torque reduction with engines with high compression ratios, improved cooling can also have a negative impact on fuel consumption because of the increase of friction and cooling losses. The new water jacket spacer with Expad controls temperature on the cylinder surface. (Earlier post.) The temperature of the middle section warms up more quickly, while the top and bottom nearly keep the same temperature. With the same water temperature, the Expad increases engine torque because of less friction loss. Ignition retarding is also minimized due to maintaining the temperature of the top section.

Resources

  • Yamada, T., Adachi, S., Nakata, K., Kurauchi, T. et al. “Economy with Superior Thermal Efficient Combustion (ESTEC),” SAE Technical Paper 2014-01-1192 doi: 10.4271/2014-01-1192

June 20, 2014 in Engines, Fuel Efficiency | Permalink | Comments (20) | TrackBack (0)

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Would adding an exhaust-gas turbine generator be worth doing?

Adding stop-start + improved transmission and reducing total weight by 30% would also help?

The asymmetric expansion of the Atkinson cycle is intended to do the same thing as an exhaust turbine.  It leaves less expansion energy in the gas.

I can't help but suspect that this is all waiting for variable geometric compression ratios.  At low power, the engine operates on the Atkinson cycle at high geometric CR.  For high power the geometric CR is lowered, the volumetric efficiency is increased and a turbocharger with a motor-generator is brought into play.  The motor-generator is used to spool up the turbo to raise boost pressure quickly, then recover excess exhaust energy for the crankshaft instead of dumping it with a waste gate.  This would allow high peak power from a very small engine, cutting weight.  The smaller engine would operate at higher average load, increasing efficiency and cutting idling losses even without start-stop.

Look at those torque/power curves--this is a *very* buzzy engine, climbing steeply to 6000rpm for peak power. I would be interested to see chart of fuel consumption, rpm at part load, which is where car engines live most of their lives.

Combining Atkinson cycle with turbocharging is nothing new. It is called the Miller cycle, or perhaps more correct, Miller system. It is a way to recover power density when an Atkinson cycle is used. With fully variable valve timing the effective CR can be varied just by closing the inlet valve early (EIVC) – or late (LIVC). Thus, the complexity of geometrically varying the CR can be avoided (there is no really good solution for this). This is also well-known. To me, it is surprising that all these features combined are not yet used. One practical problem is that single-stage turbocharging cannot provide sufficient charge pressure to achieve similar power and torque as a conventional turbocharged engine (if that high power density is necessary could of course be debated). There is less energy left in the exhaust (for the turbine) due to the high expansion ratio in the cylinder and higher charge pressure would be needed, if EIVC (or LIVC) is used, as in an Atkinson cycle. The power needed by the compressor cannot be supplied by the turbine, i.e. we have an impossible match. One way of overcoming this problem would be to use two-stage turbocharging with both intercooler and aftercooler. The intercooler (between the compressors) reduces the work for the compressors, so that the total compression and expansion work in the compressors and turbines respectively could match. Pioneering work with Miller system already more than 25 years ago on heavy-duty engines used this concept. However, two-stage turbocharging is not really something that is readily available for engines of this size. Yet, this is now common for heavy-duty diesel engines. When twin-turbos are used on light-duty diesel engines it is more of a sequential turbocharging, not “fully” two-stage, so that the small two-stage phase is just limited to the cross-over area between the small and the large turbocharger. Furthermore, the smallest engine (as I know of) with twin turbos is the 1.6-liter Renault diesel. Scaling down the technology to a 1.3-liter engine is probably not impossible but not that easy and it is definitely not readily available today.

I acknowledge the fact that Toyota is trying to pursue this route. An Atkinson engine has less power density than a conventional engine and must be scaled up to compensate for this. This reduces efficiency at part load, as also Toyota notes and which I have mentioned many times on this site. Torque dropped from 104 to 96 Nm. Due to the measures implemented, they raised the torque to 105 Nm. However, in an apples-to-apples comparison, these measures could also be applied on a conventional engine. Perhaps the gain would not have been quite as high as in this case (from 96 to 105 Nm) but my point is that one should not be misled by how the baseline was set for comparison. In addition, 105 Nm from a 1.3-liter engine is not really state-of-the-art. Another point worth noting is the increase in CR by two units. There are engines on the market with higher CR than 11.5:1. In summary, I recognize that the maximum efficiency of 38% is quite high but in reality, the “Atkinson-effect” is relatively small and the real gain in a “fair” apples-to-apples comparison would most likely be relatively small. Atkinson engines are best utilized in hybrid systems.

@E-P & Nick
Indeed, the variable geometric CR is already being done. That's what give the extra torque and power at the high rpm end. So, Atkinson-cycle is used at low to moderate rpm for high efficiency and Otto cycle is used at high rpm to maximize power to permit engine downsizing. To mitigate the risk of detonation at CR of 13.5, high rpm is used as well as increase cooling around the exhaust valve and port to lower the temperature of the hot spots.

This engine is also great for full hybrids (HEV) because it permits more engine downsizing, down to a 3 or even 2 cylinder engine, thereby reducing weight and cost, but also allowing more space under the hood for perhaps 1/2 if not all of the hybrid battery pack, thereby allowing more luggage space. This will make HEV's closer to ICEV in term of cost, weight, and luggage space and will increase market share of HEV's.

The soon-to-come solid-state battery that is much more compact and lighter than current NiMh battery used in HEV's, as well as SiC-based inverter, together with this significant engine innovation will make HEV's must-have vehicles! Very significant gain in MPG's, along with increase luggage space AND better handling due to lighter weight of the car, AND better reliability and lower maintenance cost, for purchasing cost comparable to that of a comparable ICEV will dramatically increase EV market penetration.

@E-P & Nick
Indeed, the variable geometric CR is already being done. That's what give the extra torque and power at the high rpm end. So, Atkinson-cycle is used at low to moderate rpm for high efficiency and Otto cycle is used at high rpm to maximize power to permit engine downsizing. To mitigate the risk of detonation at CR of 13.5, high rpm is used as well as increase cooling around the exhaust valve and port to lower the temperature of the hot spots.

This engine is also great for full hybrids (HEV) because it permits more engine downsizing, down to a 3 or even 2 cylinder engine, thereby reducing weight and cost, but also allowing more space under the hood for perhaps 1/2 if not all of the hybrid battery pack, thereby allowing more luggage space. This will make HEV's closer to ICEV in term of cost, weight, and luggage space and will increase market share of HEV's.

The soon-to-come solid-state battery that is much more compact and lighter than current NiMh battery used in HEV's, as well as SiC-based inverter, together with this significant engine innovation will make HEV's must-have vehicles! Very significant gain in MPG's, along with increase luggage space AND better handling due to lighter weight of the car, AND better reliability and lower maintenance cost, for purchasing cost comparable to that of a comparable ICEV will dramatically increase EV market penetration.

Correction: Sorry, I was wrong...no variable geometric CR done here, only variable effective CR done here by means of variable intake valve timing. Atkinson cycle at low speed does not need higher effective CR because faster combustion is not necessary at low speed, and would not result in higher efficiency. For peak efficiency, engine load should always be sufficiently high, around the optimal point, regardless of engine speed. The HSD will see to it that the engine will operate at optimum load.
Changing geometric CR is quite difficult and adds weight and cost to the engine and not done in commercial practice, although there are many schemes proposed and prototype built.

My experience with the Ford hybrid is that the 2.0 liter Atkinson is also very "buzzy" (I have no idea what RPMs it gets to, as the various dash displays do not include any engine speed or temperature data).

However, if the drivetrain allows the engine to get up to "buzzy" speeds whenever required, would it not be able to generate the mass-flow required to spool up a turbocharger on demand?  Diesel EGTs are quite a bit lower than gasser EGTs, so would not the exhaust energy be sufficient?

I presume that I have to explain certain aspects in more detail so that everyone understands. An Atkinson cycle has higher expansion ratio than a conventional engine. Thus, there is less energy available in the exhaust than for a conventional engine (no doubt about that, I hope…). Due to the shorter effective inlet stroke in an Atkinson engine we must also have higher charge pressure than a conventional engine (this should also be clear). Thus, we have two “negative” effects for turbocharging. (And, the higher the “Atkinson effect” we strive for, the bigger this problem becomes.) To compensate for this, we would have to increase the exhaust back pressure, which also increases pumping losses and fuel consumption. Additionally, high back pressure would increase the residual gas and risk of engine knock, unless we try to compensate by changing valve timing, which again would compromise engine performance and efficiency. However, there is also another problem. Engine manufacturers tend to utilize turbocharger to its maximum (regardless of if it is gasoline or diesel engines). Why would they not? (This might not apply to de-rated versions of a more powerful engine but I would like to limit the discussion to state-of-the-art engines.) This means that we are close to the pumping limit for the compressor at low engine speed and close to the maximum turbine speed at high engine speed. To a certain extent, we could increase the charge pressure somewhat at medium engine speed, where these limits do not apply. However, the result would still be less torque at low speed and less power at high speed compared to the conventional turbocharged engine. You could, of course put priority on either low or high speed by changing the size of turbine and compressors and sacrifice even more at one end to gain more at the other end. Nevertheless, the result would be a narrow operating speed range, i.e. engine characteristics that buyers probably would not like. Therefore, turbocharging must be developed further for this to become a viable option. (For sure, a Miller engine with lower power density than the conventional turbocharged engine could be made and it is actually surprising to me that no one does that.) I have tried to explain this problem a couple of times before at this forum but I hope it is clear this time. I want to stick to the topic, so I will not cover the differences between turbocharging of diesel and gasoline engines this time.

Peter_XX comments illustrate perfectly the Atkinson Cycle/Miller System.
Back to Otis question ("... adding an exhaust-gas turbine generator) and possibly correcting the drawbacks that Peter_XX pointed out.
The answer is yes. Recall that Formula 1 uses a electric turbo compounded system (MGU-H) to reduce turbo lag. Both Audi and Honeywell have an motor generated assisted turbocharging systems that should be in production soon.

the result would still be less torque at low speed and less power at high speed compared to the conventional turbocharged engine.

The goal of the exercise is to squeeze the same amount of peak power out of a smaller, lighter engine.  Decreasing the engine displacement also decreases pumping losses at low power.

the result would be a narrow operating speed range, i.e. engine characteristics that buyers probably would not like.

The Ford and Toyota hybrids use an "electric CVT" transmission.  The driver has no control over the engine speed or throttle setting, and never feels any effects of torque or lack thereof.  If the engine's usable RPM band is very narrow, the transmission would handle it just fine.  Achieving rated power at lower RPM would decrease the "buzziness" of the engine and improve the driving experience.  The battery can compensate for turbo lag, allowing the turbocharger to be tuned for efficiency instead.

high back pressure would increase the residual gas and risk of engine knock

Offset by a greater intake air charge.  If the boost pressure is greater than the exhaust BP (common in turbodiesels) this effect would be negative, no?

@E-P,
You're a second person after GasperG who is affected by the "buzziness" of HEV engine. To me, the Prius' engine seems to be smooth and quiet enough that its presence is barely perceptible at any rpm. If the radio (NPR) is on, or if carrying a normal conversation, the engine would not even be heard.

IMHO, Atkinson-cycle engine provides improve efficiency at low to medium engine speeds at high load, the speeds too low for turbocompounder to work efficiently due to low mass flow. Adding a turbocharger to an Atkinson-cycle engine can help increase the power and aid in downsizing further, or to increase maximum power for Autobahn cruising.

An Otto-cycle engine with a turbocompounder to harness extra exhaust energy would not be as efficient at low to moderate rpm even at high load, when the mass flow is still low.

For a given displacement, the Atkinson cycle engine cannot be boosted to the same power level as an Otto-cycle engine for reasons as Peter explained, so it remains a compromise for applications wherein efficiency is a higher priority than maximum power.

I'm sorry, why would a hybrid driver care if their Atkinson couldn't be boosted to sports-car performance standards?  As an owner, what I would like is a smaller, lighter, quieter and more-efficient engine of the same power.  Replacing the 141 hp 2-liter 4-cyl Atkinson in the Fusion with something like a 1.4-liter, 3-cyl turbo Atkinson would do the job.  The turbo muffles both the intake and the exhaust, and the transmission can let the engine rev as required to get the turbo spinning.

For a non-hybrid, Peter XX is of course correct about all the driveability issues of turbo lag and whatnot.

Whenever the discussion turns to exhaust energy recovery and the limitations of turbocharging, I keep turning back to an electrically decoupled "turbocharger", more or less the device Aeristech has suggested. Since you aren't forced to compromise the individual characteristics of axially-joined turbine and compressor components, you can optimize a turbine design for the best overall turbine/generator performance, and in turn design an electrically-driven blower that is operated as-needed to provide boost. The "accumulator" exists in the form of an ultracap bank that can provide boost from a dead start with no "lag" experience, and as the turbo-generator comes up to speed it fully powers the blower as well as recharging the caps. Modern switched reluctance technology can already provide the electromagnetics for both devices (without the need for some sort of notional anti-gravity device), with necessary switching speeds no longer a challenge for today's power electronics. E-P you essentially described a form of this with your first post.

One can reasonably posit that a capacitor bank sufficient to also provide creep power (single-digit kW for a few tens of seconds) during engine-off periods. Honestly, with non-exotic engines like the 1.0 liter Eco-Boost producing 100kW/liter today, it is not at all hard to imagine a 1.2 liter 3 or 4-cyl engine in the Camry/Fusion class of microhybrid providing very drivable, under 9 sec, 0-100km/h performance with pleasant NVH characteristics.

Roger, I do think pretty much everything you addressed ("better" batteries, whether solid state or other, SiC inverter technology, and "building in more lightness") are imminent and complementary to the engine improvements mentioned. Batteries still are the best storage medium for non-propulsion needs like A/C, steering, and other hotel loads.

Breaking the 4 liter/100km (60mpg) barrier in the current US "family" sedan in a manner completely transparent to today's driver will happen soon, and without Plug-In option. Add a very reasonable Plug-In storage (Volt-like or better) that permits most driving as EV and offers much more braking recuperation, and we are looking at a very bright set of possibilities indeed, even without a battery Nirvana.

Very interesting idea to combine start-stop and electric creep with supercaps, Herman.  If the supercaps were sized for regenerative braking as well, they would also be well-suited to source or sink power as required to respond instantly to throttle demand.  That would allow the engine proper to trade driveability for improved noise, efficiency, or other considerations.  The result could not help but be a better car.

@Engineer-Poet
I have said many times in the past that I do not want your comments but you never seem to respect that. You have taken out three excerpts from my explanation and I just have to say that you are wrong, wrong and wrong. Obviously, you have to read what I wrote thoroughly. I suggest that you read it again, again and again. I will not give a more thorough explanation this time. If you cannot understand what I have written, sorry, I cannot help you.

You are so narrow-minded that you seldom grasp when people are talking about different things than you are.  Also, if you want only to hear from people you like, you can start your own blog.

EP & Peter_XX,

Now now guys, be nice!

Once again, an argument for liquid air based propulsion, The cryogenic gas, expanding to 710 times its size allows for high variability between the required turbine speed and load. No PHEV battery is required, except as might be required of commercial vehicle accessories, and electrification of the steering, transmission, and suspension. Any turbocharger would hold excess gasified LIA, manage heat, and kick start the turbine to occasional overdrive required. Apparently the LIA systems can manage combustion without apparent worry over nitrous oxides -- one more rationale for a turbocharger/turbocompressor.

Nowhere in the prolix discussion above do I detect an answer as to the expense of automotive turbines, the serviceability, or the prospect that passenger vehicles will ever see them. This is for the trucking industry.

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