A split cycle Wankel engine

[Moreno]

What do you think about cplit cycle Wankel engine?

 

One of the major problems with Wankel Rotary Engines, is that due to the fact that one side of the housing is hot, and the other side is (relatively) cool, there is uneven thermal expansion.

This uneven thermal expansion causes distortion, which causes stress on the housing, and uneven wear on the rotor seals and on the interior of the housing. It's also a big cause of inefficiency, since gas will leak past worn seals, which reduces the effective compression ratio.

This idea is to use two separate rotors, and two separate housings. One rotor and housing is used for compressing the air or air/fuel mix, and the other acts as the actual engine. The number of power strokes per rotation is the same as if the two "engines" were separate, however, all of the power strokes occur in only one of the two housings.

The compressor has two input ports, one of which is where a normal Wankel engine might have it's input port, and the other diagonally opposite it. There are also two "crossover" ports, one where a normal Wankel has it's exhaust port, the other opposite it. To simplify engine geometry, the crossover ports are both on one face of the housing, not on the edge. It may be necessary to have check valves over the crossover ports, letting gas out but not in, but I'm not quite certain about this.

The main engine's housing has two intake ports, both on the face of the housing, and both connected (via short, sturdy pipes) to the crossover ports of the compressor. Naturally, there are also two exhaust ports -- these are probably on the face of the housing, not on the edge, but I'm not sure how important that is.

The rotors share an axle, and are slightly out of phase from one another, so that each compression event completes shortly after the main engine's intake/combustion event. The amount of phase difference determines the system's compression ratio -- a smaller phase difference results in a greater compression ratio.

The main engine has two spark plugs, located near their intake ports. Each plug fires just after the nearby intake port is uncovered. If the fuel/air mix comes in through the port faster than the flame speed, the flame cannot propagate backwards into the crossover tube, but will instead be basically stationary.

With combustion occurring during the overlap of the end of the compression event in one housing and the start of the expansion event in the other, it is nearly isochoric, which makes this similar to an ideal Otto cycle.

A stationary flame front should allow for complete combustion of the fuel/air mix, eliminating unburnt hydrocarbons from the exhaust. Also, there's no possibility, at all, of engine knock.

After the compressor's crossover port is covered by the rotor, or the check valve is closed by gas pressure, the main engine's rotor continues to expand it's heated gases, and then exhausts them.

For greatest efficiency, the main engine should have a slightly larger displacement than the compressor, so that the working gas's pressure equals atmospheric pressure just before the exhaust port is uncovered. This optional feature would make the cycle more similar to an Atkinson cycle. Besides being more fuel efficient, it be quieter.

Theoretically, the intake ports for the compressor could have a variable geometry, eliminating the need for a throttle valve, and thus eliminating throttling loss. In reality, if this were possible, Mazda would already be using it in their rotary engines. Eh, whatever.

If gasoline direct injection is done, with one injector (or set of injectors) placed in each crossover pipe, it should be easy to produce a stratified fuel charge. This would allow us to have a relatively rich mixture near the spark plug when it fires, but a lean or stochiometric mix everywhere else. This should also completely eliminate any chance of preignition. It also protects the injectors from the heat of combustion, which happens in the main motor, not in the crossover pipes.

Cooling is simpler than in a regular Wankel engine, since the compressor will be fairly cool, and the main motor fairly hot. In contrast, in a regular Wankel engine, half the housing is cool, and the other half hot.

Basically, the engine would have all the positive features of a Wankel engine, but with greater efficiency, and a cleaner, quieter exhaust. The mechanical complexity, and such things as torque, power, and redline, would be about the same as that of a pair of Wankel engines. Sealing could probably be simplified, since the housings expand fairly evenly as they warm up, unlike an ordinary Wankel engine which expands unevenly... a Mazda rotary engine has about a hundred seal-related parts, to compensate for the uneven expansion.

The compression ratio could probably be made rather higher than that of a regular Wankel engine (since this engine is highly resistant to pre- ignition and knocking), which would boost thermodynamic efficiency, and probably boost torque, power, and overall efficiency. On the other hand, increasing CR increases friction and mechanical inefficiency... testing would need to be done to determine the best ratio.

I've read that ordinary Wankel engines are sensitive to back-pressure, and don't take well to being turbo-boosted. I'm not sure why that is, but it would be a good idea to see if this engine would benefit from a turbo.

The Scuderi Split Cycle Engine [link] is prior art. And of course, so is the Wankel engine [link]!

http://www.halfbakery.com/idea/Split_20Cycle_20Wankel_20Engine

 

Will that type of engine burn oil like conventional Wankel?

If we compare free-piston generator with proposed Wankel modification (used as a generator) which one would be better?

 

 

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[Enthalpy]

This proposal isn't enough thought through.

 

First, it distorts the advantages and drawbacks of each solution in order to misguide the reader. The advantages of reciprocal piston engine are:

• Seals less extraordinarily difficult (the Wankel works, among the many rotary piston designs, because seals have been invented for it, but they aren't easy).
• Combustion not always at the same place, which permits a high temperature and keeps cooling feasible. The advantage is not much the differential thermal expansion, rather the bare feasibility. This is the efficiency advantage over a gas turbine or a rotary engine.
• Combustion at constant volume instead of constant pressure. This gives a higher temperature for the same fuel amount and improves the efficiency.

If one splits the compression and the combustion-expansion, be it at reciprocal pistons or a Wankel:

• The compression factor uses to collapse because of dead volume, and this alone collapses the efficiency.
• Combustion at a single place limits the allowable gas temperature hence the efficiency.
• Constant pressure is bad. Separate expansion permits a bigger expansion factor than the compression factor, which partly regains the disadvantage over combustion at constant volume, but a turbocharger does that better, combining the best of both.
• Reciprocating piston engines go already in this direction! They presently close the intake valve earlier in a controlled fashion so the compression factor is less than the expansion, at least at moderate power.

I should like to remind that split engines have been proposed many times and have shown every time the same known drawbacks. I suggest to check the many thousand existing patents.

 

Then, some claims are difficult to believe unless the proponents have excellent solutions ready.

• Air speed faster than flame speed, ouch! Only in an ideal world where detonation wouldn't exist. What when the walls are hot, the engine slow, and the driver requests torque?
• Air+fuel mix is the wrong choice and gives a bad impression of the designers. The obvious better method is to inject the fuel after the compression.
• Then, it can burn pretty much anything (gasoline, kerosene, Diesel oil, heavy Diesel, vegetable oil...): anything it is designer for, or at an existing engine, anything that fits the pump, the injectors, and if needed, the autoignition temperature.

So a real progress would be to burn at constant volume and expand with a bigger ratio. Just one or the other is no significant progress, and the reciprocating pistons plus turbocharger is already a partial solution in this direction.

• Maybe a reciprocating or Wankel engine can have a second expansion stage, like steam piston engines had.
• More original attempts try to add closed combustion chambers at gas turbines. Difficult, but very interesting.