When is a 4 Cylinder an 8 Cylinder?
When is an 8 cylinder motor really only a 4 cylinder? When you are talking about the Scuderi Split-Cycle Engine. The design is based off of 140-year old technology that was never completely refined. Scuderi redesigned this engine as a result of a $1.2 million contract with the Department of Defense. The concept is simple, use 2 cylinders to do the same job it would take a traditional motor only 1 cylinder to do. This may seem like a waste of energy, but use of a separate compression and firing chamber can easily be described and demonstrated. Before we got into pros and cons let’s get the theory of operation down.
The cycle begins like a traditional Internal Combustion (IC) engine, with air being sucked into the compression chamber. At this point, the first stage cylinder compresses the mixture of fuel and air. After compression has reached a maximum, a valve opens and allows the mixture to flow through a passage to the firing chamber. Almost immediately after entering, a valve seals the flow chamber and the mixture is ignited. The combustion forces the piston down, and then the spent mixture is allowed to flow out of the chamber as the piston moves back up to prepare for the next cycle.
Now lets examine the benefits for using such a process. The first and most obvious is the ability to use 2 different sized pistons for compression and firing. If the compression side is a larger bore, you will get a much higher compression ratio to use for ignition. This will produce more power and a higher combustion temperature as well as produce a much faster combustion time. The higher temperature will make for a cleaner burn. The increase in power will eliminate the need to use oversized motors.
The increased combustion time actually is a benefit in itself. Traditional IC motors begin ignition just before Top Dead Center (TDC), which actually works against the rotational force of the engine. This must be done because the combustion takes a very short amount of time to build up the pressure to force the piston down to make power. It is basically the “need money to make money” effect. The Scuderi design fires just after TDC, which removes no power from the system. This can occur because of the supercharged air combusting faster than the piston is moving away from it. Basically, the engine doesn't rob itself of power to make power, which once again, improves efficiency.
The third benefit comes from something known as the “Miller Effect,” which is the allowance of thermal over-expansion. The firing piston has a longer stroke than the combustion piston, which allows the mixture to over-expand as it combusts. This increases the thermal efficiency of the fuel and also helps to drastically reduce emissions, up to 70%. The increase in efficiency will also raise gas mileage by up to 30%, which translates to more money in your pocket and that should make everyone happy.