May 25, 2009 For more than 100 years people have been trying to come up with an engine design to supercede Nicolaus Otto's four-stroke internal combustion motor. Scuderi is the latest to take a stab, recently unveiling a prototype of a split-cycle engine that relegates the "suck" and "squeeze" strokes to one cylinder, and the "bang" and "blow" strokes to another - for a claimed efficiency improvement of up to 50%, emission reductions of up to 80% and a power density improvement of up to 70%. Meanwhile, it's also able to store large amounts of compressed air, allowing it to run as an air/petrol hybrid when cruising. President Obama's new national efficiency standards represent an opportunity for groups like Scuderi to pitch clever clean engine technology to major manufacturers - but has the split cycle engine got what it takes?
President Obama released new, tougher national gas efficiency and emissions standards yesterday - a move that aligns the rest of the country with California's strict laws. It's good news for engine efficiency innovators - major automakers will have to look carefully at all their options if they're going to meet the new standards while still delivering the torque and power the American market seems to demand.
One company that may stand to benefit is the Scuderi Group of Massachusetts, which has just produced a working prototype of an advanced split-cycle engine that promises to slash CO2 and NO2 emissions, while boosting torque beyond what diesel can deliver and increasing fuel efficiency by as much as 50%.
The Split-Cycle Engine
Effectively, the split-cycle engine splits each cylinder in a four-stroke motor into two separate chambers, each with its own piston.
Where a four-stroke engine uses four strokes of the same piston to produce a power cycle - down for intake, up for compression, down after ignition for the power stroke, and up again to push out exhaust gases, the split cycle separates the process across two cylinders.
The first cylinder is purely an intake and compression cylinder. As it travels down, it draws air in. It then compresses the air as it travels upwards - at which point a valve opens to let the compressed air mixture up into a crossover passage that pushes it forward into the next cylinder.
The next cylinder, the power cylinder, receives the compressed air charge just before the top of its stroke. Fuel is injected and the mixture is ignited after top dead center to produce a power stroke. As the piston travels back upward, exhaust gas is pushed out.
In this way, a split cycle engine is able to produce a powerstroke for each turn of the crankshaft, as opposed to a four-stroke engine which requires two full turns per power stroke.
Scuderi's Innovations
The split-cycle engine is not new - in fact, there were working split-cycle engines as far back as 1891 - but the engine configuration has been hampered by two key problems - breathing issues caused by the trapping of high-pressure gas in the compression cylinder, and a low thermal efficiency because the air was having to be re-compressed in the power cylinder.
Scuderi has patented solutions to both these issues; by firing as much as 14 degrees after top dead center in the power cylinder, the company has eliminated the second compression of the air charge, as it's still being drawn in right up until the moment of ignition.
The breathing problems have been eliminated by keeping the air/fuel mix in the crossover passage at high pressure; the compression cylinder's outlet valve doesn't open until the air within it is already at around 50 bar - and shortly after the compression cylinder's outlet valve opens, the power cylinder's intake valve opens. Scuderi describes the concept as pushing marbles into a tube: as compressed air is forced into the crossover passage, some of it is released at the other end. But enough air is stored in the crossover area that it's not pushed directly into the power cylinder on the same stroke; it might take several strokes for a particular charge of air to be fed through to the power cylinder.
Advantages
The Scuderi split cycle engine allows the intake air to be cooled AFTER compression - helping to greatly raise its resistance to pre-detonation or knock.
While a regular engine can only be boosted to around 21psi of pressure before knock begins to occur, the Scuderi split-cycle engine can be boosted as high as 36psi. The result is an engine with the torque of a diesel, that can run as fast as a petrol engine - so it delivers considerably higher power density.
A Scuderi split-cycle engine is claimed to be able to deliver as much as 70% more power than the same size gasoline engine.
Going Air/Gas Hybrid for Extra Efficiency
Beyond the basic power boost offered by the split-cycle engine, you've also got the ability to take air that's compressed while you're braking or coasting, and store it in a tank. Then, when you're cruising at low power, the engine can intelligently decide to use that compressed air to push the power cylinders down without any fuel or ignition whatsoever.
Scuderi has a number of plans for using the compressed air as a kind of hybrid backup engine, and believes that proper use of the technology can boost fuel economy by as much as 50% over a regular gas engine.
And since you've got a good on-board air compressor, you can make use of it in other ways - a valve on top of the compressed air tank can be connected to a tire inflation device or any number of other air tools - a very handy addition to a vehicle.
Prototype unveiled... Now what?
Scuderi unveiled a prototype of the split-cycle engine last month, and the company claims it plans to run the engine for the first time sometime this month, at which point all the simulations, projections and press releases will have to be backed up by genuine numbers.
While the Obama auto efficiency standards do present an opportunity for innovators like Scuderi, the company will have to make a very convincing case before any of the major manufacturers will pick it up and start using it in production models. We look forward to hearing the results of the prototype's first run, and seeing if the Scuderi split-cycle engine can live up to the potential it promises.
See Scuderi's video of the prototype unveiling below.
Loz Blain
