BP-Rio Tinto JV Files Application for Hydrogen Power Station with CCS in Kern County, California
Large Field Trial Shows Miscanthus Could Meet US Biofuels Goals With Less Land

Roush Optimizing Wrightbus Series Hybrid Drive Based on 2.4L Diesel

Wright is making both single-deck and double-deck series hybrid buses.

Engine development and engineering services provider Roush Technologies is supporting Northern Ireland-based Wrightbus in optimizing its new series hybrid drive which uses a Ford 2.4-liter diesel engine as the genset. An earlier generation of the Wrightbus hybrid drive used a GM 1.9L engine. (Earlier post.)

The Wrightbus program involves optimizing the series hybrid drive systems through a detailed analysis of generator load patterns. Roush engineers have been able to recalibrate the engine to operate at its peak performance throughout the drive cycle by using smart charging and load control technology. Overall engine performance is significantly improved when compared with normal applications.

With the base 2.4-liter engine operating as a generator, Roush engineers have been able to predict load and speed changes in advance, thus allowing greater freedom with injection strategies and EGR (Exhaust Gas Recirculation) rates.

Roush engineers have also been able to utilize some of the existing vehicle-based strategies to carry out functions which otherwise would have required significant software changes. This, coupled with a unique CAN interface module, has allowed the full integration of the engine and its controller into the overall hybrid control system at a relatively low cost.

Having the engine control as a fully integrated part of the hybrid system—and coupled with a unique calibration, has enabled us to achieve exceptional improvements in fuel consumption. During back-to-back route trials in London, these fuel savings have been in excess of 30%. The application works extremely well and demonstrates the real potential for hybrid systems… but we do get a few raised eyebrows when people realise that we are running a full size double-decker bus with a 2.4 liter engine from a light van.

—Paul Turner, Roush’s Technical Director of Product Development

Wright switched to the 105 kW 2.4-liter Ford diesel—also used in the Ford Transit and the Land Rover Defender—due to the need to exert greater control over the engine’s performance within the hybrid set up. The hybrid buses uses a Siemens drive and a lithium-ion battery pack.



Eh, pretty cool, isn't it?

That's the great virtue of a fully serial hybrid compared to a parallel hybrid.

Rather than revving the ICE all over its range of RPM and torque with complex ICE/electrical couplers, the ICE is working either not at all or full on at its optimal conversion point, loading the generator at constant power to the battery. So the loss incurred in generating electricity and charging the battery is more than compensated by the optimal efficiency achieved by the ICE in this setup.

Plus there is the benefit of a much simpler and lighter drive train and a smaller ICE. The ICE itself can also be greatly simplified as, when it runs, it runs always at the same RPM/torque point. No need for variable valves, complicated injectors, etc. Just one point of operation. The only thing that can vary is the fuel quality.

As long the battery technology (and the recharge infrastructure ) cannot match the density and performances of liquid fuel, that's the way to go for both private and urban utility vehicles.

Brian P

Fifi, I think you've misunderstood entirely.

This control system operates by NOT running the engine at a single RPM and load point.

Under appropriate load conditions, it's more efficient to sacrifice a little bit of ICE efficiency (by running it off its theoretical design point) in order to NOT have to charge and discharge the battery (a process which is approx 85% efficient). In other words, you could be off-design by something that gives about 15% increase in BSFC relative to the best-efficiency point, and be ahead of the game by letting the engine accept the load variation rather than the battery.

I don't have the BSFC map of the engine that they used in this application, but I have seen the BSFC map of a VW 1.9 TDI engine, which is rather similar. If you locate the best efficiency point (0.197 g/kWh at approx 2000 rpm and above 80% load) and you add 15% to that and identify the maximum and minimum power output points corresponding to that BSFC condition, there is a factor of about 5 in power output between max and min. Within that regime ... it's more efficient to let the engine's operating conditions vary, than it is to force it to run at a single condition and charge/discharge a battery.


I think lithium cells are more efficient than 85%..


Assume 95% efficiency for the generator and then 95% efficiency for discharging and you get 90.7% round trip efficiency.

Think about this:

For a hybrid you have two setups-
-A series hybrid where you have a .5L single cylinder engine that runs at WOT
-A parallel hybrid that has a 2.0L engine that operates in the 2000rpm range with varying degrees of load

While the second setup might achieve better thermal efficiency and perhaps better fuel economy, I would favor the first setup for a couple of reasons. First, consider that in the medium term PHEVs will probably be the name of the game. The goal is to use energy from the grid. When we do have to burn fuel it doesn't have to be as efficient as possible but instead just more efficient than we do now and at a lower cost. A single cylinder engine running flat out would weigh less, cost less, and be simpler to control.

My 2cents...



The TDI engines have a nearly flat peak efficiency curve, near full throttle at any RPM. The peak efficiency RPM is not much better than peak efficiency load at other RPM's.

The .5L will only run flat out during accel and freeway driving, meaning big fuel savings city driving.


That is one reason I have long advocated a two-cylinder opposed, two-cycle diesel as the drive of the gen-set.

For a reasonable sized car, you would only need about 1 liter.


Is it Valence inside?


Diesels do not normally have variable valves, complicated injectors, etc. so there is nothing to simplify.
A major advantage of the diesel engine is it’s “nearly flat peak efficiency curve” constant speed operation overrated in a diesel.
A series hybrid DOES have a much simpler and lighter drive train. If you make the motors big enough and the battery power high enough to accelerate the bus, adding a transmission would seem illogical.
I am not sure how Brian P knows this control system operates by NOT running the engine at a single RPM and load point. But they DO say “With the ... engine operating as a generator, .. able to predict load and speed changes ...”
I cannot imagine how you could get equivalent performance from;
“-A series hybrid where you have a .5L single cylinder engine that runs at WOT
-A parallel hybrid that has a 2.0L engine that operates in the 2000rpm range with varying degrees of load”
and then opine that “ the second setup might achieve better thermal efficiency and perhaps better fuel economy .. “
The parallel hybrid engine could be similarly small because a parallel hybrid adds the electric motor to the ICE, and charges similarly during cruise and decel by field strengthening or “downshifting” as required.
And how did a one or 2 cylinder engine get into this discussion when they are changing from 1.9 to 2.4L ? – for a bus..

A plug-in makes no sense for a bus. The “home charge” would only by good for the first few miles or less.

Mad Max

Why not use a microturbine instead of a diesel to deive the generator? It would be smaller, lighter and perhaps more efficient. Probably more expensive, but this might improve with economies of scale.



My comments were geared toward LDVs not the relevant bus conversation. Sorry.


Microturbines are the wrong direction. Even with recuperation they pnly claim 20 – 30% efficiency and weight is not nearly as important on a bus as a car. Diesel is best, gas has lower efficiency but less weight. Microturbines weigh less but cost more and have lower efficiency .


Ah, yes - for LDVs these are intersting posibilities.


It's hard to comment except when you see the decision to go to a 2.4L engine.

These are not Greyhound buses. Their route averages are typically 5mph when in downtown and 12mph in the suburbs. There is usually a couple of miles doing 30mph between those two areas. Passenger loadings are 20% in the 'burbs but can reach 80% in town as there is an overlap between people getting on and off downtown. I was a TGWU member at one time - these are my experiences from the early sixties. They have built a lot more vehicles since then but roads not so much. Has traffic sped up much ? You be the judge.

Each end of the route provides for a 10 minute layover period so the crew can recharge their batteries, so to speak. In town there could be further delays to meet another route which happened to be running late - in order to facilitate passenger transfers. Frequently off-peak you are travelling with 5% loads.

That there are Routemaster buses in London that are 50 years old tells you that the routes are pretty tame. Presumably nothing a team of four Clydesdale horses couldn't handle.

The mistake is not to think of a bus as if it were a big car that must manage as best it can in modern traffic. The fact is its physical size commands respect ; plus drivers inherently know if they hit the rear end of a bus the police will show up.

Then, because of its human cargo, its acceleration rates must be kept low for safety. The bus driver usually doesn't wait for people to be seated - this takes time - so acceleration must be limited or physical injury may occur. Of course you can circumvent that by parameterising "S" acceleration profiles into the servo amplifiers in order to allow higher acceleration rates without fear of jerking people off their feet.

All this cuts down on energy expended and eases up on the the size of the ICE required.

The first decision needed is what is intended to be the primary energy source Li-ion or gasoline.

A car should focus on gasoline like the Prius and include batteries for their power capacity instead of their energy storage. The battery package should be small for low weight.

A bus, on the other hand, which has lower dynamic performance can focus on exotic chemical energy storage in a great wad of batteries, at the same time this will supply ample power. Use can be made of an efficient donkey motor to slow the depletion of the battery energy source. The battery is not efficient in that it delivers only 90% of what went in. Since some of what went in came from the engine, this means the effective engine efficiency is but 90% of its actual value. The waste heat from the engine though is useful for passenger comfort plus the A/C compressor can be direct drive rather than less efficiently by means of a serpentine belt.

To recap, in theory therefore, you should be able to get away with quite small engines with the series hybrid technique along with a cart load of batteries, but you must accept the inefficiency of churning energy in and out the battery. This latest development from Wrightbus seeks to reduce this.

Generally speaking a bus is on the streets from 6a.m. to midnight. Let's say at an average speed of 10mph the bus needs a 180 mile range. If the pack has a hundred mile range, the parcel compartment sized engine need only make up the difference and it has 18hrs to do it. That's the PHEV model of which I am not enthusiastic. Has but one advantage - in the absence of oil we still have some sort of reduced frequency transit system running

The decision by WrightBus to go with a larger engine can be seen as an endorsement for gasoline and possibly a trend towards less electrical storage dependency. I understand that when you've committed to a 4 cyl engine the cost delta from a 1.9L to a 2.4L is likely to be minimal and as I've always said, the best way to get power onboard a hybrid is to bore out those cylinders.
At first I had difficulty understanding why a full time 120kw 2.4L motor would be required to do this job. So as ball park figures I spec'd for the the power needed at one tenth g at 30mph for an unloaded 8 ton vehicle.

Since the definitive 0 to 60 in eight seconds is 0.3 g (11ft/sec/sec), equiv accel to 24 seconds sounds reasonable for this vehicle. So I needed to estimate the power which requires one tenth g just as speed plateaus at the max service speed of 30mph. This worked out as 128 HP required with my approximated figures, so a 160HP motor should easily provide base load performance for the 12 seconds needed plus all the onboard hotel loads.

A large part of the variable passenger loading and gradeability loading should be offloaded to the battery pack to be replenished at all other times. It seems that the earlier 1.9L was falling short during acceleration thus its larger replacement.

This looks to be a good fit. If we assume constant power accel with initial S ramping to 0.3g at 10mph declining to 0.2g at 20mph and 0.1g at 30mph, this will actually be faster than the 24 secs to 30 mph given by a constant 0.1g. The extra 40Hp of the new motor will significantly minimise the strain on the battery system over the more frequent accel ramps when vehicle occupancy is light.

You need to avoid the danger of letting the engine accept too much of the passenger load variation rather than the battery otherwise you end up running an engine with significant overcapacity most of the time. this is what this tuning is about.

There may be special cases that need higher performance. I look at that as a want and not a need. In future I would hope that buses are ordered for the job they are going to do not some artificial hoop that they must all jump through. Fuel is too expensive now, it is going to be even more so in the future.

To me this points out that engineering can do only so much. It is the responsibility of the transit authorities to be fielding the optimal size of bus during the day. The easy decision to cope with varying loads by fine tuning the service frequency using the same capacity vehicle will have to be revisited. Crews should be returning the higher capacity vehicles to the depot outside of peak hours. It should no longer be acceptable to impose wear on public streets by operating low occupancy 8 ton vehicles due to lazy logistics management of the fleet, leading to excessive fuel costs to be bourne by taxpayers


At the risk of oversimplification, don't forget that any battery capacity that is not required to absorb a full stop from 30 mph is very expensive "dead weight".
The larger ICE is inexpensive and minimal additional weight.


- ToppaTom

No argument here on the new engine cost or weight as I originally wrote still "a good fit". And you probably have read my opinion of "boutique energy storage devices" like Li-ion.

On the subject of regeneration I would prefer to see the use of dynamic braking resistors. Heresy yes ! But from some photographs I've seen of disc brake rotors from the Prius the adoption of resistors to suck more than the current 10kw by means of resistors could entirely eliminate brake wear.

After I read my previous post I determined that some things could have been written better if certain information had been included in Roush's write up : -

What is not clear is whether this is supposed to be a PHEV model with the aim to seriously reduce particulate emissions in urban and residential areas and at the same time providing off-oil capability. That being so, a huge electrical power capability would exist and lead some to question the choice of this particular engine for range extension. To question the need for the continuous output of a 2.4L engine enabling a transit vehicle to maintain a service which is often averaging not much more than 5mph in some urban settings.

Or.. is the aim towards a Power Assist model similar to the Prius ? In which case the engine should be able to provide, unassisted, the power required to operate the empty or very lightly loaded vehicle. Losses churning energy into and out of its much smaller battery, except by regenerative braking of course, are therefore minimised this time.

However we need to consider what happens as the bus fills with 85 passengers of 200lbs average weight ? In the Power Assist model the intention should be to utilise battery power assist solely to provide power capacity for coping with passenger occupancy. As I wrote earlier "You need to avoid the danger of letting the engine accept too much of the passenger load variation rather than the battery otherwise you end up running an engine with significant overcapacity" and higher losses most of the time.

One could argue that moving from GM's 1.9L to Ford's 2.4L is hardly "fine tuning" but since GM may be going CH 11 after next month it gives good reason to change vendors and avoid future supply problems. And I'll leave it at that.

The comments to this entry are closed.