Is a “Split-Cycle” Engine in Your Future?

A proposed twist on the internal combustion engine could mean drastically improved fuel economy and performance in the coming years. The Scuderi Group is claiming that their prototype “split-cycle hybrid” engine offers the biggest improvement in gasoline and diesel engine design in the last 130 years with fuel economy gains of 50%.

A conventional engine makes power over four cycles of piston; hence the term “four-cycle”. In the first stroke of the piston, air is drawn into the cylinder; hence the “intake stroke”. Then the piston compresses this air on the “compression stroke”. Fuel is then introduced on the third stroke as the spark ignites the fuel/air mixture on the “power stroke”. Finally, the combustion gases are expelled from the cylinder on the “exhaust stroke”.

The split-cycle engine splits these four strokes between two neighboring cylinders. One cylinder is devoted exclusively for intake and compression, while the other is for power and exhaust. The compressed air is transferred over to the power cylinder by way of a “cross-over” tube. Because two cylinders are performing the task of one, the split-cycle engine can product spark on every revolution of the crankshaft, whereas a conventional four-cycle engine fires on every second revolution. Excess compressed air is diverted to holding tank for later use.

As unique as this may be, Scuderi is claiming that point where they have devised ignition to take place during the power stroke is the biggest breakthrough of all. An article in MIT’s Technology Review explains:

In most gasoline engines, combustion occurs as the piston approaches the top of the cylinder. In the Scuderi engine, it occurs after the piston starts moving down again. The advantage is that the position of the piston gives it better leverage on the crankshaft, which allows the car to accelerate more efficiently at low engine speeds, saving fuel. The challenge is that, as the piston moves down, the volume inside the combustion chamber rapidly increases and the pressure drops, making it difficult to build up enough pressure from combustion to drive the piston and move the car.

The split-cycle design, however, allows for extremely fast combustion—three to four times faster than in conventional engines, Scuderi says—which increases pressure far faster than the volume expansion decreases it. He says that fast combustion is enabled by creating very high pressure air in the compression cylinder, and then releasing it into the combustion chamber at high velocities.

Here is video that illustrates the concept:

Will this engine design revolutionize the auto industry? It’s still too early to tell. As mentioned in the Technology Review piece, nine major auto manufacturers have signed nondisclosure agreements with Scuderi Group and about $65 million has gone into this concept. Despite this, so far the only data gathered has been based on computer simulations. It will be interesting to see if this fascinating concept delivers real world results.