The RCCI engine uses both gasoline and diesel fuel

When most of us pull up to the pump, we are in vehicles that require gasoline. A portion of us sidle up to diesel pumps. In the future, maybe we’ll we’ll drive vehicles with RCCI engines and we’ll need to hit both pumps.

The Reactivity Controlled Compression Ignition (RCCI) engine is a concept engine being developed by the University of Wisconsin-Madison, and it only exists in the lab. What it’s capable of, however, could be revolutionary. Our friend Jason Fenkse from is the perfect person to break it all down.

The RCCI engine can deliver up to 60 percent thermal efficiency. That means the engine will convert 60 percent of its fuel into power. That’s a massive leap in thermal efficiency compared to the average road car. It’s 10-15 percent better than the average diesel engine. As for gasoline engines, the Toyota Prius’s engine will hit a remarkably high 42-percent thermal efficiency, while the Mercedes-AMG F1 engine reaches to 50 percent. Most other gasoline engines have thermal efficiencies in the 30-40-percent range.

To do this, the RCCI engine makes use of two types of fuel. It needs both a low-reactivity fuel like gasoline and a high-reactivity fuel like diesel. Other fuels are possible, but these are the most common fuels. Reactivity is a fuel’s willingness to combust.

The way the RCCI engine works is a bit of a dance in the combustion process. Like a standard gasoline engine, a mixture of air and fuel is sprayed into the combustion chamber. This occurs through a port injector during the intake stroke as the piston moves down from top dead center. That’s typical. At this point, however, the combustion process becomes unique.

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During the compression stroke, a direct injector mixes diesel fuel into the chamber, creating a mix of gas, diesel, and air. As the piston gets closer to the top, a bit more diesel fuel is injected into the mixture and this sets off ignition. This is called a cool flame because it’s not very hot. The ignition causes the diesel and gas mixture to ignite and that causes the remaining gas to ignite.

The result is a more efficient fuel burn, and a cleaner one to boot. Even the NOx and soot emissions from the diesel fuel are kept very low because the fuel is well mixed.

While that all sounds great, this setup isn’t without its drawbacks. The largest drawback is the need for multiple and separate fuel systems. That means separate fuel lines, fuel tanks, and fuel fillers. That also means filling up twice, once with diesel and again with gasoline.

For now, the RCCI engine represents an intriguing concept. Click play on the video above to learn more about this rather unique engine concept.

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