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.
Having a separate air-compression cylinder makes it easy to divert compressed air into a storage tank, which can have a number of advantages. For one thing, it’s a way to address one problem with gasoline engines: they’re particularly inefficient at low loads, such as when a car is cruising at moderate speeds along a level road. Under such conditions, the air intake in a conventional engine is partly closed to limit the amount of air that comes into the engine—“it’s like sucking air in through a straw,” Scuderi says, which makes the engine work harder.
In the new engine design, rather than shutting down air flow, the air intake is kept wide open, “taking big gulps of air,” he says. The air that’s not needed for combustion is stored in the air tank. Once the tank is full, the compression piston stops compressing air. It’s allowed to move up and down freely, without any significant load being put on the engine, which saves fuel. The air tank then feeds compressed air into the combustion chamber.
The air tank also provides a way to capture some of the energy from slowing down the car. As the car slows, the wheels drive the compression cylinder, filling up the air tank. The compressed air is then used for combustion as needed.
It is still far from clear whether the design can be a commercial success. Even if the simulation results translate into actual engine performance in a car, the engine may not prove to be easy and affordable to manufacture, Rinek says, especially with equipment in existing factories. The design will also have to compete with many other up-and-coming engine designs. Scuderi says the first application of the engine might not be in cars, but instead as a power generator, especially in applications where having compressed air on hand can be useful. For example, construction sites can require electricity for power saws and compressed air for nail guns.