Today’s highly contested topic of discussion is the most recent attempt at turning the 5-stroke engine into a feasible reality. Yet another unused engine that was capable of 150 brake horsepower from just 0.7 liters of displacement, and was both 20-percent lighter and 10-percent more efficient than its closest competitor.

Marketed for being “100-percent conventional” in its architecture, the 5-stroke we speak of was recognized for having the fuel economy of an intercooled turbo diesel, but without all of the particulate emissions or silly additives. First teased at the 2009 Stuttgart Engine Expo (now called the Automotive Testing Expo), the 5-stroke prototype from Ilmor Engineering received a moderate amount of press, but also a lot of scrutiny.

 The company’s co-founder and partial owner, Roger Penske, has long been in the limelight, so anything associated with Ilmor tends to get a ton of press regardless of how many strokes it might or might not have. These ultra-deep racing roots have long provided Ilmor with the ability to provide race-ready powerplants to the likes of Honda, GM, and Mercedes-Benz in Indy Racing, F1, and so on, along with the funds needed to explore wonky engine porotypes like the modern 5-stroke. 

This leads to two opposing questions: Was the forsaken 5-stroke engine complete rubbish? Or was it just a bit ahead of its time and needed some more engineering incubation before being allowed to hatch?

So, we’ll provide the details, you bring the comments, and let’s see where the conversation goes…

Deutsch Diesel Designs Done Differently

Ilmor wasn’t the brains behind this motor’s development, but merely worked with a German designer by the name of Gerhard Schmitz to further explore a 5-stroke engine prototype design. Schmitz wasn’t the first to pursue this uneven kind of combustion either, with the massive (and massively inefficient) Daimler Otto-cycle design being the first accredited 5-stroke on record.

Unlike Daimler though, Schmitz and Ilmor were able to use modern computer software to monitor the average expansion ratio of their group effort and compare it to a diesel engine, which typically sits somewhere around 14.5:1. Weighing a hell of a lot less and taking up far less real estate, Schmitz’s design seemed like a solid bet after initial testing. Furthermore, the concept’s commendable output levels and low-pressure cylinder design simultaneously made the 5-stroke far more efficient than the conventional 4-stroke, at least when pushed to maximum output levels.

While this 5-stroke concept used the old 4-stroke cycle for “High Pressure” (HP) implementation, it differed in that it benefitted from an extra cylinder for further expansion. With the addition of this central “Low Pressure” (LP) cylinder, the other cylinders could now dump any pressure that was left over post-power stroke into this scavenging chamber. Once the piston reached bottom dead center, the exhaust valve opened, and out went the rapidly expanding hot gas and all of the exhaust crud that comes with it.

But instead of funneling this spent energy straight out the exhaust, the 5-stroke took these spent gasses and cycled them a third and final time. As the piston within this low-pressure cylinder was pushed down, an additional fifth stroke was formed, thus allowing the crankshaft the extra 180 degrees of powered rotation it needed to produce more power. Meanwhile, the scavenging of these spent fumes and the addition of a small but extremely efficient turbocharger system reduced emissions and boosted fuel efficiency ratings. 

Running of the concept engine has produced impressive fuel consumption readings over a very wide operating range. This is because at the onset of knock, a greater percentage of work can be extracted in the low-pressure cylinder, giving a degree of self-compensation. — Ilmor Engineering

Oil Burners and Baby 4-Strokes Beware

So precisely how efficient and forceful was this little turbocharged 5-stroke?

According to Ilmor, the 0.7-liter engine that emerged from the exploration of this platform provided brake-specific fuel consumption figures in the 226 g/kWh range, which at the time was a 10-percent improvement over modern four-stroke designs. The 5-stroke engine also offered fuel consumption and emission levels comparable to that of modern diesel engines from that era, but without all of the unpleasant particulates, NOx emissions, and additives associated with “oil burners.”

This was all thanks to that extra cylinder providing an additional expansion process with a ratio rivaling that of a diesel engine but on a miniature scale. Allowing that extra stroke to provide some additional elbow grease allowed for greater scavenging, combustion, thermodynamic, and filtration efficiency ratings, and gave Ilmor the data it needed to move forward with dyno testing.

With its revised 5-stroke ready for action, it seemed like the Ilmor/Schmitz engine was poised to outperform everything on the market at the time, and they now had the figures to support these claims.

During testing the turbocharged 700cc 5-stroke Ilmor engine made 130 horsepower (that’s 185 ponies per liter!) and 122 lb-ft. of torque. Granted, these figures were produced in a lab where the engine could operate at optimum controlled temps in ambient air, but go figure. Apparently, limited testing was completed on a motorcycle at one point too, but not much was released to the public in regard to the results of this experiment.

“Poof” Where’d the 5-Stroke Go?

So, if all of this revolutionary engineering was so spectacular, why aren’t we seeing it in operation today?

Last we heard, Ilmor was saying that the prototype was ready for road testing and could be used either as a conventional engine or as part of a hybrid drivetrain. It was also in the hunt for a partner to help further develop the engine and was in talks with automakers and suppliers in the hopes of securing investors.

But then there was nothing but radio silence, and the whole 5-stroke radar went dark for a prolonged period of time.  Updates beyond the flurry of media blitzes surrounding the reveal in Stuttgart and the following press releases from Ilmor explaining its intentions, were nowhere to be found.

It is rumored that this was due to the engine’s creator, Gerhard Schmitz, taking a new job with another auto manufacturer (thus raising patent issues), and the no-compete legal paperwork, likely bogged the entire project down.

Over time, key hyperlinks within Ilmor’s press releases began to disappear as well, and today, there is not a single mention of the 5-stroke engine design on the Ilmor Engineering UK company history profile page, nor the Ilmor USA website. It’s almost like the entire project was whitewashed over in the hopes that we wouldn’t recall what once was or could have been.

But some of us haven’t forgotten, as the linked video clearly illustrates.

Despite misspelling the inventor’s name in the intro of the video, YouTuber Repairman22 was right on about pretty much everything else in their video synopsis. Schmitz did secretly continue his research, and with the help of French engineers, developed a new, offset 5-stroke engine design with an integrated wastegate among many other improvements. The project just never got anywhere further than that, at least as far as we know.

And then there was that Polish university engineering project that converted a 2.0-liter Volkswagen Golf motor into a 5-stroke design. By modifying and machining out the center of the block, the engineering team was able to reconfigure the two central cylinders so that they could function as one large low-pressure cylinder. This proved that pretty much any engine with a traditional cylinder and piston design could be modified into a 5-stroke, and did not require many additional parts to make it all happen. This testing also proved that the 5-stroke engine benefited significantly from water injection, as heating issues had been a concern on previous 5-stroke experiments.

Unfortunately, problems with optimum efficiency, increased internal friction levels, and ideal power outputs only being achieved under maximum throttle plagued this design as well. There were also concerns over poor catalyst filtration due to those lower exhaust temps, and a gratuitous amount of turbo lag was consistently noted, regardless of what size or design of snail was affixed.

All of those extra moving parts and turbocharged accessories were also a consideration. Not only did they negate the majority of the 5-stroke’s power-to-weight ratio figures, but from a production standpoint they added a ton of cost and assembly time. The whole unit also required a fair deal of engine bay real estate, and due to its complexity, was revered as quite the convoluted mess to maintain and tune, hence it never getting further than lab testing. As one commenter stated after viewing the video, “This is basically a turbocharger turned inside out.”