Published on December 6th, 2008 | by Ryan Douthit21
Rotary vs. Piston Engine Equivalency
Comparing rotary engines to their piston counterparts has probably ended more than one friendship and perhaps even led to minor bloodshed. Such is the nature of apples-to-oranges arguments, but why the disproportionate fuss? Rotaries are not well understood by most people—too frequently including those that rightfully sing its praises—and nothing devolves into a pissing match like an uninformed argument! Equivalency is a hotly contested subject, not only in actual racing but more importantly in bench racing. Here, we look at the dimensions of equivalency between the two engine types.
The most common rotary engine, the 13B, is rated at 1.308 liters (~80ci) of displacement, yet produces power out of proportion to its rated size, well in excess of the magical 100 hp/l mark; indeed, it hits 182 hp/l in current non-turbo RX-8 trim! Rotary owners can be forgiven for bragging. However, this does not tell the whole story.
Displacement is the swept volume of each cylinder (or chamber, in rotary terms). This is its theoretical capacity for moving a volume of fluid (air/fuel) over one cycle, were you to look at it as an air pump – and you should, because the volume of air/fuel throughput is proportional to the power output of the engine with given efficiencies. Total displacement in a piston engine is simply cylinder displacement times the number of cylinders. Sounds logical, but from the perspective of an air pump only half that volume can be moved per revolution due to the mechanics of a four-stroke piston engine, where one cycle requires two revolutions. So, the capacity of a piston engine is half total displacement.
In a rotary engine, displacement is a bit trickier. Each rotor has three faces, or working chambers, and there are two rotors in a 13B engine. All six chambers have a displacement of 0.654 liters, but it takes three revolutions of the output shaft to bring all those chambers through a full cycle. Were it to be rated by the same method as a piston engine (chamber displacement times number of chambers), it would be called a 3.9 liter, which is hardly logical. The accepted approach is to treat each rotor housing as a cylinder and measure the displacement of one chamber per rotation. Interestingly, this happens to be precisely equal to capacity. In other words, on every revolution of the output shaft, the rotary engine moves a volume equal to its rated total displacement.
For the purpose of displacement equivalency, either the rotary engine total displacement needs to be doubled or the piston engine total displacement needs to be halved. The latter approach is more correct (equates with capacity) but the former is simply more politically correct, as this is a piston engine world and those exaggerated ratings are just too familiar to our culture. So, rather than say that a 2.6-liter piston engine has 1.3 liters of capacity (true), we say that a 1.3 liter rotary has equivalent displacement of 2.6 liters (also true, but back-assward).
Because rotary and piston engines differ so much in mechanics, it is no wonder they have very different efficiencies. Here, there are two main areas for analysis. First, there is Thermodynamic Efficiency and, second, there is Volumetric Efficiency.
Thermodynamically, the ideal combustion chamber is a sphere. A piston engine is closer to this ideal shape, so it is no wonder why it’s the more efficient design in this respect. However, it also has to reciprocate, involving wholesale changes of direction and lots of surface area friction, and there is a quite involved valvetrain mechanism to operate—meaning there are hundreds of parts dancing in synchronicity just to make the thing work. Two steps forward, one step back. In the final analysis, the prime measure of thermodynamic efficiency, Brake Specific Fuel Consumption (BSFC), gives the edge to piston engines. In other words, per each unit of fuel consumed, piston engines tend to turn a little bit more into useable power than rotary engines sporting relatively long, thin combustion chambers. Where a piston engine can be presumed to be .50 to .55 BSFC, the rotary is traditionally around .60 BSFC. Volumetrically, however, rotary engines are superstars. Without a valvetrain in the way and while breathing through relatively large ports, rotaries tend to better fill their chambers and thus realize a higher proportion of theoretical capacity. So, while being less efficient per unit of fuel consumed, rotaries generally compensate for it by moving more air/fuel per unit of rated capacity. Another area of consideration is that this good breathing is advantageously used at higher RPMs to yield high horsepower without necessarily high peak torque, so a rotary engine’s torque has a longer lever, so to speak (i.e. mechanical advantage of RPM). This good breathing also takes place over a relatively wide RPM range, so the torque tends to be very linear, which certainly helps drivability.
While rotaries, as discussed above, tend to be less thermodynamically efficient, much of that wasted heat is actually carried in the exhaust gasses; more so than with a piston engine. Turbochargers, by nature, convert thermal energy to kinetic energy–supplying the impetus for turning the compressor wheel–so rotaries can salvage a great deal of that otherwise lost energy. Rotaries, for this reason, are often turbocharged.
While it is hard to draw too many comparisons, a few stand out. First, the rotary engine is, indeed, a four-stroke motor. Each chamber has four distinct strokes per cycle, just as in a conventional automotive piston engine. The difference is that each chamber moves within the rotor housing, passing over the intake and exhaust ports rather than requiring an actuating valvetrain. Just like a 4-stroke piston engine, the intake opens near Top Dead Center (TDC) on the intake stroke and closes near Bottom Dead Center (BDC) as the chamber goes into compression, the air/fuel is then ignited at TDC to start the power stroke, and finally the exhaust port opens near BDC and closes near TDC to complete the exhaust stroke. Two-stroke piston engines, on the other hand, open their ports only near BDC and every such time. Rotary engines do require oil injection to lubricate their apex seals (similar in purpose to piston rings), but that is due to the fact that adjoining chambers share a seal and there is no other way to deliver the lubrication. Use of injected oil in no way makes the rotary engine a two-stroke, contrary to some ill-informed opinions to the contrary.
A rotary engine can be thought of–in some respects–as a large-bore, short stroke-piston engine. While the rotor does not reciprocate in a traditional sense, the relative motions of the rotor and housing effectively expand and contract the working chamber volume as if it were a piston in a cylinder. This is best illustrated by nulling out the rotor rotation and observing one working chamber as the housing rotates around it.
The mechanical Achilles heel of the rotary engine is the exposed, unsupported nature of its apex seals. Because of this, rotaries are susceptible to detonation from lean air/fuel mixtures and overly aggressive ignition timing. Naturally aspirated, rotaries are very disinclined to detonate without quite a lot of motivation, but in turbocharged applications, tuning is critical to prevent breaking apex seals. Still, a properly prepared and tuned turbo rotary can make in excess of 500hp with decent reliability. The problem is that most people are too cheap or too poorly educated to do it properly, so the reputation of the engine has suffered. In fairness, the proclivity of a rotary engine to break apex seals from clumsy tuning is not that different than the propensity of piston motors to break parts when over-revved. Either is a no-no, but the latter case is far, far more ingrained than the former.
Overheating is also something rotaries also do not tolerate well. However, on the plus side of the column, the greatest feature of the rotary engine it is remarkably compact external dimensions, comparing closely to that of a beer keg. This small size and low weight allow the engine to be mounted very low and far back in the chassis of a car, yielding perfect 50:50 weight balance. Given the power density and tight packaging, it is no wonder rotary engines enjoy a very strong racing legacy.
Rotary and piston engines can be precisely equated on some fronts, like volumetric displacement, but issues of thermodynamic and volumetric efficiency cloud any true formula for how many liters of displacement of one type of engine produces equivalent output to some displaced volume of the other type of engine. In other words, we can say a 1.3-liter rotary engine is factually the equivalent displacement to a 2.6-liter piston engine, but that does not make them necessarily equal in horsepower output or torque characteristics. This is why racing sanctioning bodies tend to either use oddball numbers for equivalency that have only superficial relation to volumetric equivalency (e.g. 1.7:1), or utilize some other means, like spec carburetor chokes, spec porting or alternate engine size, to level the actual performance, as typically done in SCCA. True equivalence is elusive, but volumetric equivalence is simple math: 2:1.
Article by Blake Qualley, first printed in Driving Sports Magazine, January 2004.