Split Cycle Engine: True or Hoax?


This is a so-called Scuderi split cycle engine, which claims 135 hp per liter, and much higher efficiency than normal engine. Two cylinders work together. One compresses air which is passed to the other cylinder after TDC, and the other one is the firing cylinder.

Is this another hoax or could there be something to this concept?

Probably not a hoax, but that doesn’t mean it’s a real, practical, and economic alternative. I’ll wait and see, espcially regarding emissions. As for 135 hp/liter, conventional four strokes can get that, the latest motorcycle engine exceed that.

Here’s another alternative configuration - also real, but yet to be proven practical:

It’s claimed that carbon dioxide emissions are reduced by 50%. I can understand reducing emissions by reducing fuel consumed, but the claimed fuel consumption decrease is 36%. It claims a reduction in oxides of nitrogen of 80%. As I understand it, oxides of nitrogen are not an inevitable result of gasoline combustion, but carbon dioxide is.

I’ve heard about reducing carbon dioxide emissions, but never understood how it would be done. Perhaps someone familiar with the chemistry involved could explain how the emissions can be reduced.

Engines that burn hydrocarbon fuels like gasoline, diesel, propane, natural gas etc, yield an end product that is water (H2O) and carbon dioxide (CO2) under perfect conditions. No engine operates at 100% efficiency so you get other stuff like unburned hydrocarbons (soot), carbon monoxide (CO) and other junk, which is why you need cat converters and other emissions controls. The only way to reduce CO2 emissions is to burn less fuel to begin with. They may have mis spoken and meant they reduced CO emissions, which is possible with better efficiencies. They do state in the article that “This technology has been explored before without any meaningful success.”

When you burn fuel in air like the stuff we breathe you get NOx, period. Air is over 70% N of various flavors mainly N2. Combustion temp determines most of the NOx mix. It is always there however. So it is an inevitable product of gasoline combustion. If you burn carbon in any form you get COx. You also get unburned fuel. On the emission test this is called HC. This happens due to flame front issues. The flame needs to move through the fuel air mix before the pressure drops due to the piston motion(this changes the reaction rate). In a lab under perfect control of the engine rpm you can optimize the flame front for the rpm. In real life we have starts and stops. And from this point car emissions gets really complicated. So thats the short version.

What I meant, more precisely, is that oxides of nitrogen are not an inevitable product of gasoline combustion, in that there is a way to burn gasoline and not derive that product. That would be done by using oxygen instead of air. That’s impractical, but my question was about chemistry, not practicality. I also meant that I know of no way to burn gasoline that does not produce carbon dioxide, or even produces less carbon dioxide.

I don’t understand how an engine can reduce carbon dioxide more than it reduces fuel consumption. Reducing unburned hydrocarbons would have that effect, but I thought that unburned hydrocarbon emissions (from the engine, pre-cat) were already very low and had little room for improvement. I may be mistaken in that.

Right now, it doesn’t really exist because it isn’t in production. In four or five years it might save us from hybrids which seem to be doing well despite my dislike for them.

With all the reversing of mass in pistons, cranks, counterweights, valves, how can that even come close to a turbinengineffiency?

You understand things well. An engine can’t reduce CO2 output by a greater amount than it does fuel use (ignoring CO production, which is quite small, anyway). So that mistake in their claims makes me more suspicious of their other claims.

This could be done if you both increase economy and switch fuels to a lower-carbon fuel, such as natural gas. But I didn’t get the impression that’s part of the plan.

Turbines are considerably less fuel efficient than reciprocating engines, unless they run in multiple stages (look up compound turbine).

Gasoline contains a certain amount of carbon. The engine either burns it or not. When gasoline is burned, it can produce either CO or CO2. I prefer an engine that produces more CO2 because that implies that it is burning gasoline efficiently. Incomplete combustion is less efficient and would produce less power. Increased CO implies incomplete combustion, too.

I read the article, and I’m still skeptical. It appears that they produce fewer emissions by using less fuel. An unbiased peer review of the engine would provide useful information.

On the emission test this is called HC. This happens due to flame front issues. The flame needs to move through the fuel air mix before the pressure drops due to the piston motion

Don’t forget the areas where the flame front cannot access. These collectively are called total crevice volume and represent a significant contribution to HC production.

It would appear that the secondary piston is being used to boost intake pressures to the primary cylinder. A small supercharger would seem to be a more efficient way of doing this. Both are driven directly by the crankshaft, but a supercharger has no reciprocating parts. Reciprocating parts waste energy.

Those are my thoughts.

Why does reciprocating motion waste energy?

When the Wankel engine first appeared on the automotive scene, high efficiency claims were based on the flawed premise that a conventional engine’s reciprocating motion wasted large amounts of energy, yet, Wankels were never more efficient than good old piston engines.

The truth is that it does take a large amount of energy to accelerate a piston from a dead stop to its maximum speed, but, when the piston decelerates back to zero speed, all that kinetic energy is returned to the crankshaft. Think of a clock pendulum. It reciprocates but it takes very little energy to keep it going. The kinetic energy at the middle of the swing becomes potential energy at the end of its swing. The energy is constantly being converted from kinetic energy to potential energy and then back again. The only energy loss is air resistance and bearing friction.
Likewise, a piston engine loses very little energy due to reciprocating motion.

A moving mass left unadultrated by an outside force will just continue on its path, never changing speed. It does not resturn to zero speed unless an additional force causes it to. It takes energy to get the piston to speed, and it takes additional energy to change its velocity back to zero. That’s the way inertia works.

With today’s lightweight piston and connecting rod materials, the amount of energy used isn’t as large as it used to be, but it’s there.

Rotating mass has inertia also. But once a rotating part is spun up, it’s not slowed down again. The initial energy used to accelerate it remains in the mass as inertial energy. Its only enemy is the friction of its bearings and the resistance of the (in the case of a supercharger) air that it’s moving.

ceramic engine? Where is that

TSMB answer explains why, when you downshift, there is far more engine braking effect. The higher RPM generates much higher inertia loses. The term “compression braking” is an urban legend. Since the throttle is closed, there is nothing to compress. The vacuum acting on the piston is the same in both directions…

The clock analogy is cute but incorrect. The swinging pendulum never gives up it’s energy…It stores it and returns it with every swing…With a reciprocating piston, it comes to a complete stop twice each revolution, and at those moments contains no stored energy. The clock is aided by gravity, the piston engine is not…

In a 4-stroke engine, the piston must start and stop 4 times but is only being driven one time…The exhaust, intake, compression strokes all take power and that power must come from somewhere…

It is piston speed that ultimately limits engine RPM…Big bore / short stroke engines are always able to produce more power because less power is absorbed by inertia loses. All other things being equal…

I look for the Wankel engine to make a comeback now that direct injection is possible, eliminating the emissions problems…That and manufacturing cost was the big issue with the Wankel’s…

But lets face it…The big issue with I.C. engines is heat loss, thermal inefficiency. They work much better as a hot-water heater and hot air furnace than as a producer of torque…

When a piston slows to a stop, its kinetic energy doesn’t simply vanish, it has to go somewhere. Energy cannot be created out of thin air or destroyed, the basic law of conservation of energy.

What happens to the energy a moving piston has when it slows to a stop at the top and bottom of the stroke?
The same force applied to the connecting rod that slows the piston is also acting on the crank pin. As the piston approaches bottom dead center, the force that pushes upward on the piston to decelerate it also pushes down on the crankpin resulting in the engine’s flywheel accelerating. The piston’s kinetic energy went from the piston to the flywheel. As the piston goes back up, the force needed to accelerate it now slows the flywheel back down as the flywheel returns that kinetic energy back to the piston. On single cylinder and inline fours, there is a measurable cyclic speed variation of the crank due to this effect.
With six and eight cylinder engines, the crank is accelerating one piston at the same time it is decelerating another piston and instead of a cyclic crank speed variation, the crank simply transfers one piston’s kinetic energy to another piston.

Engine braking is mostly due to pumping losses as the engine pumps air from the partial vacuum of the intake manifold to the atmospheric pressure of the exhaust manifold.
Two stroke motorcycle engines had nearly zero engine braking when the throttle was closed because two stroke engines generate much lower intake manifold vacuum than four stroke engines do. Diesel engines are also notorious for having little or no engine braking. These engines never restrict airflow through the engine with a throttle. Some truck engines do have a throttle in the exhaust that is closed just so the engine will act as a brake when going down long grades.
Another way diesel engines are made to brake is to change the valve timing so the engine compresses the air but the exhaust valves open way too early and dumps the pressure so the compressed air cannot return its stored energy to the crankshaft on power stroke. This also makes the exhaust extra loud.

I look for the Wankel engine to make a comeback now that direct injection is possible, eliminating the emissions problems…That and manufacturing cost was the big issue with the Wankel’s…

But lets face it…The big issue with I.C. engines is heat loss, thermal inefficiency. They work much better as a hot-water heater and hot air furnace than as a producer of torque…

I think the biggest problem with Wankel engines is that they have so dang much combustion chamber and piston surface area for their displacement. It’s like the world’s shortest stroke/biggest bore cylinder.
The less surface area the combustion chamber/piston crown has, the lower the thermal losses. Every BTU of heat that migrates from the hot high pressure gasses to the engine parts is a BTU of heat that the engine will never be able to convert into 778 ft-lb of mechanical energy when the piston expands those hot gasses.
If you look at modern engines, you will notice that the “old fashioned” long stroke engine has made a comeback as fuel efficiency and emmissions has become a priority.

For what use ? The tooling already committed to present day autos even makes it difficult to displace the pinto based engine from the 80’s and used in the ford ranger “forever”. If it’s worth it’s salt, you’ll see it first in outboards, snowmobiles and maybe hybrids, then and only when Asians do it first… Good conversation, look for it first in a toro.