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Three Mercedes-Benz Citaro G BlueTec Series Hybrid Buses now in Service at Stuttgarter Straßenbahnen

CPT/AVL Study: New Generation of Micro-Hybrid Technologies Can Enable More than 25% Reduction in Fuel Consumption at Lower Cost Than Full Hybrid Drive Systems

Reductions in fuel consumption and CO2 from base 2.0-liter ELC-Hybrid  to  downsized  1.4L  VTES  +  SpeedStart. Click to enlarge.

The mild electrification of cars using a new generation of low-voltage micro-hybrid technologies—including an electric supercharger, belt-integrated starter/generator, and a carbon-enhanced  advanced  VRLA  UltraBattery (earlier post)—can enable  existing  technology  engine  and  transmission  combinations  to  be  aggressively  downsized  and  downspeeded  to  support  very  significant  (>25%)  reductions in fuel consumption and CO2 emissions,  while  maintaining  acceptable  levels  of  driver  enjoyment, according to a study by Controlled  Power  Technologies  (CPT) and AVL presented at AVL’s Motor und Umwelt conference last week. 

The paper was jointly authored by CPT engineering director and chief technical officer Guy Morris with Mark Criddle, Mike Dowsett and Toby Heason from CPT and Dr. Paul Kapus and Matthias Neubauer from AVL.

CPT and AVL have  been  working  for  the  past  18  months  to develop a  value-driven  micro/mild  HEV  solution,  utilizing  CPT’s  production-ready  VTES  electric  supercharger (earlier post).  CPT  and  AVL’s  most  recent  work  assesses  the  potential  of  a  VTES  equipped  downsized  ELC-Hybrid  vehicle  in  combination  with  the  CPT  SpeedStart  Integrated  Starter  Generator (earlier post) and  the UltraBattery.

  The  intent  is  to  enable  aggressive  yet  near  term  downsizing  and  downspeeding  of  existing  engine  families,  delivering  proven CO  reduction and fuel economy improvement,  without  the  usual  dynamic  compromises  that  typically  limit market acceptance. 

—Morris et al.

Based on a VW Passat family-sized saloon, the demonstrator currently incorporates a advanced, AVL-developed 2-liter gasoline engine already delivering a fuel consumption of 6.6 liters/100km (36 mpg US) and CO2 emissions of 154g/km—a 20% reduction from the 192 g/km of a series production Passat (MY 2006) fitted with a 2.0L TGDI engine and close to the 146 g/km of the current Passat 2.0L common rail TDI engine. In effect, AVL has developed a gasoline engine with diesel-like fuel efficiency.

One of the significant features of this demonstrator is its long gearing ratios to enable down-speeding of the engine. Normally this would result in unacceptable high gear vehicle acceleration, but the integration of CPT’s variable torque enhancement system or VTES provides an important dynamic performance boost, reacting instantly to transient load conditions by delivering up to 25 kW of additional power to the crankshaft in less than a second, even at the lowest engine speeds.

The next technology step is to further develop AVL’s efficient low carbon ELC-hybrid concept by incorporating CPT’s SpeedStart system. In combination with the VTES electric supercharger, this will create a cost effective micro/mild hybrid system which, when applied to a downsized 1.4-liter variant of the ELC-hybrid power unit, will meet the European industry’s 130 g/km CO2 emissions target.

The use of a carbon-enhanced valve-regulated lead-acid (VRLA) battery will help maximize energy recuperation during deceleration, fully realizing SpeedStart’s potential for high power generation and hence electrical energy recovery.

The combination of low voltage micro-hybrid technologies incorporating stop-start, brake regeneration and electric boosting—as well as exhaust gas regeneration which we’re also working on for the longer term—can help minimize the additional cost of CO2 compliance to the consumer to between €1,500 and €4,000 within the 2015 to 2020 timeframe. This compares with €6,000 to more than €18,000 for a full hybrid or electric vehicle.

—Guy Morris




Well known, mature, low cost technologies that could have been implemented 1+ decade ago but were not because car makers obviously wait till governments force them to incorporate fuel saving changes.

Is it too much to say that car manufacturers, specially the Big-3, seem to be induced by ............... to produce higher fuel consumption vehicles.


Market system theorists would say that the public would want higher mileage cars and the car makers would produce them. This is apparently not the case in the U.S. over the past 20 years. Higher mileage and lower fuel consumption is good for the country and the individual, but it seems to take a mandate to make it happen.


An ingenious integration of the obvious using, as Harvey puts it "Well known, mature, low cost technologies".
This concept would work extremely well in the problemantic segment of pickups/suvs, ie the heavier vehicles.
Hopefully we will see this in production soon.


If it is, it would probably be a different model. There are those that just want their V8 and poor mileage is a right. Anything else is just big government trying to run their lives.


Nothing new here: By running the ICE in a narrow efficient rpm range and using an electric motor to boost acceleration, you have a parallel HEV...the Prius is an example.


I know I don't let up, From the designer/ technicians point of view,

I'm seeing more complications added to unrefined existing technology.

It becomes ugly.Feels like I have one foot on a station platform and the other on the train.
Although there is no reason for me to suggest it wont meet the substantial claims, They do seem to be heading towards over complication.
I've said before that If these systems are to be diagnosed and serviced by the (average) school 'dropouts'(everyone starts somewhere) otherwise known as motor mechanics, they should start with clean sheet elegant and easily deciphered design.

The real world production line offerings are a compromise, some pull it off to a remarkable degree and in themselves work well, but there are as many existing technologies that are elegant need fewer complex areas of expertise that are intreracting with each other in fairly complicated ways to understand and diagnose.
This really matters if maintenance and repair are ever going to be incorporated. This is an area of real concern frustration expense and avoidable fuel waste, emissions and safety risk to us motoring public.


On that subject I'm reminded of a lecturer saying
"If they designed those jumbo engines to be worked on by monkeys - and they could, we'd (you all) will be in real trouble" IE out of a job. Unless of course we were capable of doing things that only monkeys can do!


If there is an engineer who is knowledgeable in car engine design i got question : why don't we just design a diesel engine that runs on gazoline, it would be 25% more efficient right away and no need to do new development just adjust the timing for the injection.

Also why don't they develop 2 cylinders 2 strokes gazoline direct injection engine with scavenging pump and on which you can deactivate one stroke out of 2 at low load (with a system like the multiair that can keep the valves closed) . Should pretty economical and simple as well as efficient ?

Roger Pham

Well, Honda's IMA (Integrated Motor Assist)scheme is even simpler and more reliable as well as cheap as the BAS (Belt Alternator-Starter) system, but Honda's hybrids do not sell nearly as well as Toyota's more expensive and complex but more efficient HSD (Hybrid Synergy Drive) system.

The 3rd gen Prius is now very affordable, with MSRP of about ~22,000 USD for the base model without much of the bells and whistles. Still, the base-bottom Prius III still is loaded with enough features such as automatic climate system and power everything that this price is competitive with many non-hybrid 5-seat sedans having similar interior space and comfort level.

Gasoline evaporates too quickly yet requires higher temp to auto ignite than diesel fuel. This means that at low power when the fuel-air ratio is very low, the gasoline vapor is too well mixed and is too diluted for flame propagation in compression-ignition mode. (ie. engine will misfire) You would need even higher compression ratio than current diesel engines...and diesel engines already are much heavier than gasoline engine of the same displacement due to the sturdier construction. If you use spark ignition on gasoline engine with as high compression ratio as a diesel engine, you will have much higher risk of engine knocks (detonation). HCCI is an attempt to bring diesel-level of efficiency to a gasoline engine...but requiring significant level of complication in engine sensors, monitoring, and control of exhaust valve timing, etc, and can run only at a narrow and low power setting. Watch out for a future HCCI-capable vehicle at your nearest GM, or MB, Nissan,... etc dealers...

Most auto mfg's have developed 2-stroke gasoline direct injection engines in the 1980's. All these are abandoned due to difficulty in emission control system and the gain in specific power and fuel efficiency weren't sufficient to warrant further development. Fiat is now offering the Multi-Air two-cyclinder but Four-Stroke engine that is very efficient.

Roger Pham

Furthermore, Treehugger, many auto mfg's have developed stratify-charged spark-ignited gasoline engines running lean with higher compression ratio, trying to compete with diesel's efficiency. The problem is in NOx emission control in a lean-burning engine that can't use a lower-cost and very effective 3-way catalytic converter system. NOx control in a diesel is a lot more expensive than in a conventional gasoline engine.


Roger, Thanks I understand for the diesel, so you would need a variable compression ratio engine or switch to a spark ignited regime at low load, or like mercedes with its Otto diesel.

for the 2 strokes, why does they have problem of emission control, even in direct injection mode they are not different from a combustion point of view from 4 strokes engines with direct injection ? are they ?

How about HHCI with 2 strokes configuration, HHCC works at low to moderate load, so in a 2 strokes you can limit the load due to the fact that there is twice as much strokes ?

Roger Pham

Problem with direct-injection 2-stroker is in the lack of precision of air-fuel control. In a 4-stroker, the amount of air entering the intake port is what remain in the cylinder. In a 2-stroker, the entering air is used to purge out the exhaust air, and some amount is invariably lost in the exhaust. This lost of air is varied and cannot be controlled precisely, because some of this air is mixed with the exhaust.

Since 2-stroker is not a viable as far as emission control, then 2-stroke HCCI is moot.

Since diesel fuel has 11% higher energy content on a volume basis, then the 25% increase in mpg is down to 14% when gasoline is used in a diesel engine. A VW Golf TDI costs $2500 higher than the gasoline version for this ~14% gain in thermal efficiency. A Prius III also costs $2500-$3000 higher than an equivalent gasoline vehicle, but can get 75-100% gain in fuel efficiency. Seems like full HEV is a better bargain as far as fuel efficiency.
On the other hand, the TDI Golf has faster acceleration and is more fun to drive than the Prius III, but the Golf gets 34mpg instead of 50 mpg of the Prius.


It is possible to burn gasoline in a diesel engine, or preferably, a fuel somewhat similar to kerosene. This would be a fuel with octane and cetane number between diesel fuel and gasoline. It could be produced with higher energy-efficiency than current mix of diesel and gasoline. To avoid knock at high compression ratios, direct injection must be used, as in a diesel engine. For ignition, a spark plug (spark assisted diesel engine) or a glow plug must be used. Preferably it should be a glow plug, since the high compression ratio and high turbocharger pressure would reduce the spark plug life. Is this a new idea? No, it was tested in the so-called Texaco engine already some 25 years ago. There are many other examples. Note that the Detroit Diesel 2-stroke bus engine for methanol (and ethanol), conceived in the late 1980’s, could also run on gasoline. This engine used glow plugs and internal EGR to reduce glow plug power. Today, we have ceramic glow plugs that last the life of the engine and common rail injection systems with very high flexibility. Thus, the technical problems experienced in the past could be overcome. Emissions would be similar as with diesel fuel, implying that a DPF and NOx catalyst would have to be used to meet tough emission limits. This would be an expensive solution, just as diesel engines are today. However, it would definitely be technically feasible to run gasoline in a diesel engine. The problem is that nobody wants to do that.

Note that the "hypothetical" engine above has nothing to do with HCCI (which is another option, by the way...).

Roger Pham

Thanks, Peter, for further clarifying the issue.



You ask a very serious question.

Engineers from many automakers are doing the R&D to do precisely waht you suggest.

Homogeneous Charge Compression Ignition, HCCI, is the mouhtful of technical phrases that we engineers use to describe a semi-diesel that burns gasoline and is compression ignited like a diesel.

It has the benefits of both gasoline so-called Otto cycle IC engines of comparativly light weight, clean emissions, and smooth operation, with the increased efficiency and fuel economy of a diesel.

It turns out that HCCI cannot work at all speeds and loads, so the effort is going forward to expand the time you can do so, and revert to regular spark-plug ignited ignition for the times when you can't.

Engineers are getting very close to success, too. It was speculated that the Opel Insignia,(our Buick Regal), would be released with an Ecotec 2.4 Liter engine running as an HCCI at least some of the time.

It missed and didn't happen. It was delayed and would have been the first mass production HCCI engine. GM has demonstrated this engine to the Press, and let reviewers drive the prototype cars, so its not very far aways from happening.

One of the bright features of Fiat-Chrysler's Multiair engine technology is that it accomplishes over 95% of the work necessary to convert an ICE engine to an HCCI power plant, very economically. Chrysler is putting Multiair on it WGE I-4s, and its brand new Pentastar V6s as well. It is starting to build small Multiair I-4s from Fiat, the "FIREs" at a brand new factory in Michigan too, for the Fiat 500s sold in the Americas. So about 75-80% of its engines will soon be HCCI capable, with half the weight of diesels, and almost the fuel economy of them.

As an example, the Cummins I6 used in Ram pickups weights almost 1100 pounds. The Pentastar v6 weighs barely 400 pounds. While not exactly comparable, it reveals the extremes of weight savings that can be had.

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