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Alan Taub: GM pushing on split-cycle engine technology in the lab; looking for low-load efficiency and packaging

Noting that split-cycle engine technology “really looks promising”, GM Vice President of Global Research and Development Dr. Alan Taub said that the company is putting on a “major thrust” in its R&D laboratory to see if it can get split-cycle technology “moving”. He made the remarks during the introductory plenary session today at the DOE’s 17th Directions in Engine-Efficiency and Emissions Research (DEER) Conference in Detroit.

Taub noted that improved technology is enabling engine makers to go back to options to improve engine efficiency that have been talked about for decades, but were not then possible to realize. One of those potential high-efficiency options is the split-cycle engine, which separates the compression and expansion components of the combustion cycle into discrete cylinders.

Today the split cycle is still in the laboratory, together with some collaboration outside of GM. The promise of dramatically increased efficiency is what we are looking for. There are two prongs to this effort. Can we get the peak efficiency north of 50%, and more importantly, can we really move up the efficiency at low loads, which is where that technology is really challenged? And how tight can we get the package and the weight and the incremental cost?

The driving force is clear, we can see dramatic improvements in efficiency by going to the DCDE [discrete compression discrete expansion] architecture. It’s definitely something we need to explore further.

—Alan Taub

In September, the Scuderi Group released findings from a computer simulation study measuring the performance of the Scuderi split-cycle engine modeled against the European class of “high economy” vehicles. The data showed that a turbocharged/air-hybridized Scuderi Engine can achieve at least 64 mpg US (3.7 liters per 100 km) while emitting 85 g CO2/km. (Earlier post.)

Another split-cycle start-up, Tour Engine, reports that a simulation of its opposed-cylinder design run by an independent consultant—and verified by a major OEM—suggests that a feasible configuration of the TourEngine could achieve about 56% brake thermal efficiency (BTE). (Related post.)

Taub also noted that GM was pushing ahead with its development of HCCI (homogeneous charge compression ignition) engines (earlier post), and would be exploring application in downsized turbocharged engines such as the new 1.4L Ecotec (earlier post).



I have been advocating, "downsized turbocharged engines", for more than ten years. A two-cylinder opposed diesel, air cooled, driving a gen-set would get 100 mpg+ in a reasonable weight auto. It would only need a small battery pack to boost passing and merging acceleration.

The transmission would be as simple as an Aluminum wire.


Will GM do what they should have done decades ago or will somebody else do it.


This does appear to deflate the nay-sayers.

Roger Pham

Split-cycle engine idea was invented by George Brayton even before Niklaus Otto invented his Otto-engine. In the last 100 years, there were countless attempts to revive the split-cycle concept for piston engine...none could replace the simplicity, power-to-weight ratio, and reliability of the Otto-cycle engine.

Best wishes to GM and Scuderi efforts.


Roger, in the last 100 years all that people wanted from their engines was simplicity, power-to-weight, and reliability. Now they also want fuel economy so now is the right time to replace the Otto-cycle engine. The real question is 'replace it with what?' The energy efficiency, simplicity, power-to-weight ratio, and reliability of the electric motor - or - the range of some more complex fueled system.


Is it too late to change ICE design and raise the efficiency from 25% to 50+%?. If it can be done without extra cost, why not. A down sized version could be used in extended range PHEVs for the next 2 or 3 decades or until such times as FCs and BEVs take over.


The Scuderi Technology is the most developed and advanced Split Cyle engine in the world and several OEM's know it. SWR has been developing this engine for many years with staggering results and all of a sudden this small company out of W.Springfield Ma. is no longer a laughing matter. The American OEM's can not afford to delay moving on this groung breaking technology and it appears they just took a jump to the head of the class.


There are further improvements on the basic Scuderi cycle. A big one is not to cool the air in the crossover/storage system (use regenerators if possible) and expect fuel to ignite and burn immediately when injected. The expansion cylinder's piston and head can be insulated with e.g. ceramics to cut heat losses.

It's probably too complex to regenerate exhaust heat to pre-heat the air going through the crossover, but something like Transonic Combustion's supercritical fuel injection system can recycle exhaust heat back to the combustion chamber through the fuel. If there's enough diluent in the fuel to eliminate hot/rich spots and the NOx/soot they create, this could be close to perfect.

Thomas Pedersen


The most important feature of a split-cycle engine is to expand the gases down to ambient pressure and therefore close to ambient temperature. Consequently, the exhaust temperature of the expansion stroke should not be higher than the exit of the compression stroke.


Not a student of thermo, are you?

Expanding gases to ambient pressure won't get close to the intake temperature.

  1. Even considered as an ideal gas, the temperature is increased considerably in combustion so it cannot be reduced to the same value without being expanded through a much larger ΔP.
  2. There's an entropy increase in combustion which creates an unreducible amount of waste heat.
  3. The ratio of specific heats of CO2, etc. is lower than air, which reduces the cooling effect of expansion.
This heat can still drive vapor engines, thermoelectrics, absorption A/C, etc.

Roger Pham

Even though it is always exciting to work on new engine concepts, GM should put their "major thrust" in R&D laboratory to make their HEV's and PHEV's more competitive with those soon-to-come from Ford, Toyota, Huyndai, Honda, etc.
Reason? Retail price of Lithium Iron Phosphate (LiFePO4) battery is now down to $450/kWh retail, and even lower at whole-sale level (?$350/kWh?)

With cycling performance of 2000 cycles to degrade performance down to 80% of capacity, and calendar life of over 10 years, LiFePO4 battery can be practically discharged even 4000 times in a PHEV before requiring replacement. At a cost of $350/kWh-capacity for 3000 cycles, the cost of battery is 11.6 cent/kWh of battery electricity. Adding to this 10 cent/kWh retail cost of electricity, and each kWh in a PHEV will cost but 21.6 cent, multiply by .85 efficiency battery to wheel, and the cost will be 25 cents/kWh at the wheel.

A conventional ICE vehicle at 18% thermal efficiency tank to wheel at $3.50/gallon of gasoline will cost 59 cents/kWh at the wheel. PHEV is already competitive with ICE today, at lower over energy cost while attaining energy independence.

The Kia Optima hybrid Guiness Book of World Records 64mpg done by the pulse-and-glide method while driving close to the speed limits is a real eye opener. It means that current HEV still have too large an engine. If the Optima hybrid is to be equipped with a 2-cylinder 80 hp engine coupled with a 80 hp electric motor and 2.8 kWh battery, one would expect much higher mpg out of it, probably a combined mpg of ~60 mpg...using current technology already on the shelf.
Install a larger battery of about 4-6 kWh to this 60-mpg HEV, and one would have a real potential for a 14-20 mile all electric range. Enough for a daily commute, yet still affordable to many people.

The Toyota PHEV Prius with its 4 kWh battery and 14 mile all electric range is something that GM should focus their effort on.
A new and improved Chevy Tahoe hybrid may use a 4-cyl 2.4 Ecotec Atkinson at 160 hp plus a 160 hp electric motor, instead of the 320 hp V8 now. Forget about the too expensive 2-mode hybrid system, and use the simple Huyndai hybrid scheme, instead. The Ecotec will have a CR of 13 to achieve Atkinson effiency when running on gasoline, using delayed closure of intake valve. Perhaps an ethanol/water injector will be provide for use when peak power is desirable, when no delayed intake valve closure will be used, to avoid power loss associated with Atkinson, to boost output to over 200 hp for brief peak output. Since the Tahoe has plenty of internal space, a 10 kWh LiFePO4 battery can be installed for ~15 mile AER range, to be charged at work to obtain 30-mile AER.

For God's sake, GM, make the Tahoe and Suburban more aerodynamic...forget about the squarish male-macho look...adopt a more curvaceous contour...Gentlemen love curves!!!

Petroleum independence using existing technology.


Many good points RP. Unfortunately, the male-macho competition is still very strong within the pick-up groups. GM will eventually have to give in and try to catch up with the competitors with more efficient vehicles. Otherwise, GM may need Government hand-pouts again in a few years.

Thomas Pedersen


Please read what I write before resorting to insults!

I said that by full expansion to (close to) ambient pressure, the temperature will not be higher than *after* the compression stroke. Therefore there is no point in trying to recuperate heat - a process which always increases complexity and incurs aerodynamic losses.

In a split-cycle engine, compression and expansion strokes can have different pressure ratios. So, aside from entropy generation during combustion, friction, etc. you actually could expand to near-ambient temperature. I doubt that friction, non-isentropic conditions etc. will result in significantly more temperature increase than compression with a pressure ratio of e.g. 11 (isentropic temperature increase of 222K).


Your knowledge and commentary on new engine technologies is commemorable, however when it comes to the Split Cycle engine you seem to be a little closed minded. If you get a chance maybe you can look into the Scuderi technology with more of an openmind and come back with your findings. I can't see why several OEM's would be this interested in the split cycle potential, if there was no possibility of being more efficient and have more economical advantages than the Otto cycle engine of today.


Thomas, do a little homework. The polytropic gas law (Pv^γ = C) is good enough for ballpark figures.

Try CR = 11.0, γ = 1.4 for the compression.
Increase temperature by 1000 K in combustion, in a constant volume regime.
Then expand back to atmospheric pressure with γ = 1.27.

Post your results and say if they suggest that your scheme would work or not.

Roger Pham

Split-cycle principle is being applied daily on every gas turbine engines flying overhead and on every gas turbines in power generation, and even on the Capstone recuperated-heat micro gas turbine. The engineers know all the ins and outs of split-cycle to the minute details.

The problem in applying split-cycle to piston engines is in the valves and the heat involved and in the improvement of efficiency well over what Otto and Diesel cycle can offer. Gas turbines do not have to worry about valves and the lubrication requirement of piston, piston ring and cylinder. Gas turbines that give out comparable power to a piston Otto-cycle and Diesel-cycles engine fall short in thermal efficiency. The recuperated-heat Capstone turbine with foil bearing and optimized to the minute details for efficiency can muster 25-30% efficiency at best at 35-50 kW output level, far below what a comparable Atkinson cycle engine at 37-39% and a small turbodiesel at 42%.

All Scuderi group need to do is to demonstrate prototypes that can run for thousands of hours for the same or less maintenance cost, equal or higher reliability, and much higher efficiencies than the best of existing Otto-cycle or Diesel-cycle engines, and the world will be better served. I'm nobody and my opinion is just that, opinion of a single person.


The Scuderi prototype has been running for over 2 years with around the clock research and developement by many top notch engineers at Southwest Research. There are many new advances on their split ctcle technology which unfortunitely you are not privy too. However the Scuderi Group has been publishing information via white papers, maybe they can answer some of your concerns on the reliability of the split cycle engine.

Roger Pham

Please kindly provide a link so everybody can see.


Since Thomas hasn't come back in 4 days, I'll do his homework to show the public why his claim is false. (It's obviously false on 2nd Law grounds, but it's best to do it with math instead of handwaving.)

Given the polytropic ideal gas law Pv^γ = C, P = Cv^(-γ) and T is proportional to v^(1-γ)

Compressing air (γ = 1.4) by 11:1 volume ratio from a starting temperature of, say, 310 K yields a final temperature of 809 K (536 C) and a pressure ratio of 28.7. Increasing the temperature by 1000 K increases the pressure ratio to [28.7*(1809/809)]=64.2. Expanding by a pressure ratio of 64.2 with γ=1.27 yields a final temperature of 419 K or 146 C. (I think these figures are right, but using Windows Calculator it's hard to be sure.)

In practice, viscous and other losses force more energy to be exhausted as heat rather than being converted to work.

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