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Concept Engine: Ilmor Engineering to Show Cutaway of 5-Stroke Engine at Engine EXPO

Rendering of the Ilmor 5-stroke concept engine. Click to enlarge.

Ilmor Engineering will present a new cutaway version of the Ilmor 5-stroke concept engine at the upcoming Engine EXPO 2009, 16-18 June, in Stuttgart, Germany.

Ilmor is a UK-based independent engine design, development, testing and manufacturing company, founded in 1984 by Mario Illien, Paul Morgan and Roger Penske. It traditionally has focused on racing engines, although its business is becoming increasingly diversified, with recent projects in production automotive industries (including diesel), among others. It is also a member of the UK Low Carbon Vehicle Partnership.

The 5-stroke concept engine is a self-funded proof of concept. It uses a novel design to achieve a diesel-like expansion ratio on spark ignition—the high effective expansion ratio provides improved fuel consumption.

The 3-cylinder, 700cc engine delivers power output comparable to a 1.8-liter four-stroke engine, with minimum fuel consumption of 226g/kWh, with less that 240 g/kWh for more than 30% of BMEP. The engine generates better fuel consumption than the latest downsized, turbocharged GDI engines, and is optimal for use in an range-extended electric vehicle application, according to the company.

The 3-cylinder engine uses different-sized cylinder bores.

There have been several “five-stroke” engine designs proposed and patented. One, invented by Gerhard Schmitz, proposes the use of at least one larger low-pressure cylinder functioning in a two-stroke mode located between two smaller high-pressure combustion cylinders, functioning in a four-stroke mode.

The design decouples the compression ratio and the expansion ratio by adding a second expansion cycle for the exhaust gases by means of the third cylinder. The five-stroke cycle of the Schmitz engine thus includes:

  1. Intake into the smaller high-pressure combustion cylinders;
  2. Compression in the combustion cylinders;
  3. Combustion and expansion in the combustion cylinders;
  4. Exhaust from the combustion cylinders and intake/expansion into the larger low-pressure cylinder; and
  5. Exhaust from the low-pressure cylinder.

The overall expansion ratio will be the product of the volume ratio of the first expansion and the volume ratio of the second expansion.

The work chamber of each combustion cylinder in the Schmitz engine communicates with the work chamber of the low-pressure cylinder via a decanting valve and manifold. The volume compression ratio of the combustion cylinders is relatively low, and can be highly supercharged.


  • Ilmor Overview

  • US Patent number: 6553977, “Five-stroke internal combustion engine”



Assuming gasoline density of 750g per litre, and a 90% efficient generator attached, that 226g/kWh figure would mean 3.0 kWh electricity produced per litre of gasoline, or 11.2 kWh per US gallon.

Assuming 5 miles per kWh for a modern PHEV, that would equate to 56 mpg (US) in series-hybrid operation, or 67 mpg (UK). Not bad, but other engine types can extract more energy from the same amount of fuel for better results in this type of application.

(The chemical energy of gasoline is about 9.7 kWh/litre, meaning peak efficiency at about 34%).

Roger Pham

The operation principle of this engine is the same as the Atkinson-cycle engine as used in the Prius, Ford Escape hybrid etc., with similar results, with the Prius engine a little bit more efficient.

However, this engine is simpler, with fewer cylinders and valves, and potentially a little bit lighter, with the larger over-expansion cylinder having less internal friction and heat loss in comparison to two smaller cylinders in the Prius' engine. Engine balancing is equivalent to the 4-cylinder Atkinson cycle.


Isn't it a bit late to improve ICE from 20% to 30%?

Future PHEV-100+ will not use the onboard genset that often. It could be a very low cost average performance ICE unit or a small fuel cell.

By 2015-16, BEVs will have enough e-range to satify most requirements.


Thanks Clett, I was just breaking out my "mental calculator" when I saw your post.

I seem to remember somebody claiming 42% efficiency on a range extender but I just can't remember where I saw it now or what type of engine it was.

Can anyone point me to one they can find in this range?


Besides the usual thermal problems, did they even think about free inertia forces and free moments of inertia? Looks like a pain in the a...

Roger Pham


The engine's balance is pretty much the same as a conventional 4-cylinder engine. It will be just as smooth.

There will be a problem with increase heat in the combustion cylinders that has to work harder now with higher pressures, while having less expansion, thus increase heat built-up, hence engine's longevity may be adversedly affected.


Roger that Roger, same balance as a four cylinder but with 25 percent fewer parts. Longevity shouldn't be too much of an issue as this unit was designed for a PHEV. If said vehicle travels 200k miles in its life, the engine may only incur 50k miles of use, all of which would be at (nearly) constant load.

Did anyone catch if this unit has FI? If not a simple single scroll turbo could be slapped on while reducing the size another 200cc to 500cc (at which point both of the combustion chambers would be 250cc, the same as if this were a 1 liter I4, I don't think they count the middle cylinder in their measure of displacement as it is only used for expansion).

Roger Pham

Well, as you're pleased, GreenPlease.
However, some Prius (Prii) are used for taxi services, which see over 300,000 miles only after a few years.
Due to the extremely competive automobile market, extended reliability is a necessity in order to stay competitive. Further size reduction would not be advisable. For that reason, the new Prius 2010 now has increase displacement to 1.8 liter, up from 1.5 liter.


To all

Ilmor’s five stroke concept is actually nothing else than the Schmitz five stroke engine.

Look here, for more details


There are many ways to gain in efficiency.
This design as I understand it is to lower compression to allow high boosting to move the operating range out of the throttled area of the engine map. To gain back some efficiency loss by lowering the compression and with out utilizing the prius design back flow or some other complex valve design is to utilize an expansion cylinder. A relatively simple design to get Atkinson. With everything you add on to an engine to gain here or there there is a cost or some drawback. This design adds an extra cylinder, which could have been used to gain in capacity. I think it is an elegant and simple solution to a problem. Yes it not the only one, but this is his and quite unique one at that.
I hope you get what you deserve Gerhard.

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