When is a 4 Cylinder an 8 Cylinder?

I have 8 cylinders….but only 4 make power…?

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.

Soources: ScuderiGroup , Wikipedia and the Autochannel

George Delozier
Guest Blogger

Oct 8, 2007
by Anonymous (not verified)

power of 8 with only 4

Nice comment, however I believe there is a glaring inaccuracy whne you say with the help of the governemnt. Cool

Oct 8, 2007
by Anonymous (not verified)

To further explain the

To further explain the previous comment, The Scuderi Group was awarded a contract for the development of the engine but for various reasons declined it. The Scuderi Group has used the contract as a vote of confidence from the DOD in order to bolster its position in the engine community.  The Scuderi Group has been developing several forms of the split cycle. The reference link to the Scuderi group can explain this further.


Oct 9, 2007
by aa (not verified)

Haven't we see this before?


I enjoyed reading this article, as well as others by this author. 

I think that it was MAZDA that created a Miller 5 stroke engine in late 80s-early 90s. This company has been taking some very clever risks with the development of engines which is usually an expensive process. They are also firm supporters of the beautiful Wankel engine. The highlight of their Miller engine was that it used a very efficient compressor to "add 1 stroke" to the classic otto cycle.

I actually favor the 4 cylinder version because of less friction. When one adds another "block" with 4 cylinders relying on gas expansion perhaps the engine does not end up as efficient as it would with a re-routing of the gases to the same cylinders.

Oct 15, 2007
by SuperTuneMike (not verified)

initail impressions on the 4/8 cylinder design

I can see both benefits and engineering challenges in this design.
Having two different chambers for intake and compression and another for combustion and exhaust allow for some facinating options. Experimenting with bores and stroke of each cyclinder, shape of the chamber, head design (for each cylinder and it's purpose), transfer port shapes and types, different means to control the transfer and pressure in the charged mixture, temperatures in the intake side vs the power cylinder, ....
Further development might include a type of forced induction for intial or transfer of the charge, variable valve timings for different conditions, and a mirad of other factors.

I wonder about the complexity of doubling the mechanical parts, and the associated friction, and losses due to heat and other factors. Concerns for weight and several other factors come to mind.


Nov 13, 2008
by Anonymous

anoopchand comments on miller cycle in scuderi engine....

iam a mechanical engg student,i liked this article...i know,in conventional otto cycle engines,miller effect causes some loss of power due to removal of some gas during first part of compression stroke...but in scuderi engine,no gas is removed but it is allowed to expand more in combustion cylinder by increasing the combustion cylinder length....i feel it wonderful.....