Piston Vs. Rotary

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Rotary vs. Piston Know Your Enemy By Jim Mederer, Racing Beat

This article's topic will be a discussion of some of the distinctive features of a Mazda rotary when compared to a production-based four-stroke reciprocating engine. Much of the comparison will be technical, but some will be based on opinion - my opinion, since I'm writing this darned article!

To begin, for simplicity, we'll compare two 1-cylinder engines and we'll only talk about one of the rotor flanks--there are three.

All rotational angles are quoted for the output shaft (eccentric shaft/crankshaft), not the rotor. Both engines burn a compressed fuel-air mixture to develop rotational power. Both are four-stroke engines.

However, one big difference between them is that the recip has 180 degrees per stroke (or 4 x 180 = 720 degrees per thermodynamic cycle) while the rotary has 270 degrees per "stroke" (or 4 x 270 = 1080 degrees per thermodynamic cycle). Yeah, you may have to think about that one for a bit, but trust me, it's true.

This has some good and some bad consequences. Assuming that both engines have similar maximum rpm's (and I think that is roughly true), it means that the rotary has 1.5 times as many milliseconds to accomplish each "stroke". This is one reason why rotaries breathe so well - they have more time (in milliseconds) to draw in and spit out the mixture.

They also have more time for the power stroke - a real plus to get the most out of the combustion gas, especially at high rpm. Now the bad part. The rotary also has 1.5 times as many milliseconds to transfer heat from the burning mixture into the oil and water.

This is one reason why rotaries waste more heat in the process of staying cool. Another consequence is that, if you only consider one flank of one rotor, the rotary only gets 2/3 as many power pulses as the recip. However, there are actually 3 flanks to each rotor, each at a different point in the thermodynamic cycle, so each complete rotor actually gives 2 times as many power pulses (3 times 2/3) as a 1 cylinder recip.

Put another way, a 2-rotor rotary has the same number of firing pulses as a 4-cylinder recip, but, because the DURATION of each firing pulse is 270 degrees, the engine runs smoother due to the overlap of the firing pulses.

OK, so what is the point of all this math? Well, the point is to get a better understanding of WHY certain things are so important to a rotary - especially heat transfer. Remember, heat is potential power, so keeping heat in the combustion mixture makes more horsepower you can use.

On to the next item: In comparison to a recip, the intake charge (once it is inside the engine) actually travels a long, tortured path.

In a recip, the center of gravity of the intake charge only moves an inch or two as the piston moves back and forth between top dead center (TDC) and bottom dead center (BDC). In Mazda's rotary, the charge moves a long way - more like 20 inches - from intake to exhaust. One bad result is that there are a lot of square inches of surface through which to transfer heat, reducing thermal efficiency. However, here is the big point: The entire mass of the intake charge must pass through the narrow area between the rotor housing and the rotor as each rotor flank passes through TDC. This is made possible by the "rotor depression" which is cast into each flank of the rotor - if it weren't for that path, the partially burned mixture would never be able to squeeze through the narrow clearance between the rotor housing and rotor (usually around .010~.015 inch) at high rpm. There is a crude parallel with a recip that has a "pop-up" piston that tends to cut the combustion chamber in two at TDC. Some recips even cut a "fireslot" (notch) in the middle of the pop-up area to prevent it from stopping flame front propagation in the chamber. For this reason and others, the shape of the rotor depression is quite important. It also has a major influence on determining the compression ratio of the engine and, as all the "Internal Combustion Engine" textbooks point out, the compression ratio is a major determinant of the power and efficiency of any engine. Actually, this points out a weak point in the rotary - the maximum PRACTICAL compression ratio is not determined by detonation (as is common in recips) but by the ability of the burning charge to pass through the rotor depression! If the depression is too small, pressure builds up in the vicinity of the Trailing spark plug causing NEGATIVE WORK! This can reduce power, overheat the Trailing spark plug, and substantially increase the heat dumped into the oil and water. Therefore, the shape of the rotor depression is a cut-and-try balancing act to find the best compromise. Before we leave the subject of the rotor depression, one more point - The physical shape of the depression at its leading edge has a lot to do with the maximum usable Leading ignition advance. You can understand this better if you set a late-model rotary at 35 degrees BTC, take out the #1 leading spark plug, and look into the spark plug hole (a mirror and light might be helpful). What you will see is the curved flank of the rotor rather tight up against the bottom of the spark plug hole. If the spark plug were to ignite at this point, the engine might well misfire because the flame front might be snuffed out (quenched) when it hit the rotor surface.

If you now turn the engine to 20 degrees BTC, the way is open to burn into the mixture in the rotor depression. This is an important part of the reason why nearly all 1974 and later engines can run no more than 20 to 25 degrees ignition advance at high power (earlier USA model engines had a very long, shallow depression that allowed more advance). As I explained earlier, there are some parallels between rotaries and recips here - combustion chamber and piston top design are major concerns in recips - but there are some distinctive items to consider when working with rotaries.

The truth is, there isn't a lot that you can do to change the shape of the combustion depression, especially in 1989 and later engines with thin casting walls, but you can do some useful things. For one thing, you can ensure that the distance from the apex seal groove to the leading edge of the combustion depression is the same distance on all flanks of all rotors so that all will tolerate the same ignition timing (grind the leading edge of the depression as necessary).

Next, you can try to reduce heat transfer into the rotor by polishing the combustion depression and/or coating it with a "heat barrier" coating (Note: Do not add any measurable thickness to the curved flank of the rotor, otherwise the rotor may hit the rotor housing). Many recip racers do the same sort of things to pistons and combustion chambers, for the same reasons.

I know it isn't easy for those of you who are not deeply familiar with rotary engines to wade through this information, but if you don't understand these basic concepts, other matters (like port timing and ignition timing) will make no sense later on.

I'll give you one more item to think about - the spark plug. Books can (and have) been written about rotary engine ignition, so I'll only touch on one area - the heat range. For those who don't know it, rotaries tend to use very cold spark plugs - that is, plugs that cool their electrodes well through the water jacket. There are many reasons for this, but one of the most obvious is that, while the reciprocating engine has burning mixture around its spark plug for a nominal 180 degrees (the power stroke) out of 720 total degrees (or 25% of the thermodynamic cycle time), the rotary has burning mixture around its leading spark plug for roughly 70% of the cycle time.

Since it gets so little "cool-off" time, it must be cooled through the water jacket. This is not really the case with the trailing spark plug - it only has burning mixture nearby for 25%~30% of the cycle time, similar to a reciprocating engine. Other circumstances cause it to get a lot of heat input - but we'll save that for another time.

Until then, THINK! Rotary engines do some marvelous things. Try to understand them and use them to your advantage. You'll get through the quarter mile faster and enjoy it more!

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