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Introduction to Engine Tuning: Rotary vs Piston tuning

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Rotary vs Piston tuning

04.16

00:00 - If you're a rotary engine enthusiast, you're probably wondering how the tuning basics and the tuning process applies to the rotary engine.
00:10 While the inner workings of the rotary engine are vastly different to a piston engine, the way the rotary engine responds to fuel and ignition timing is actually remarkably similar to a piston engine.
00:23 So, most of what we can apply to tuning piston engines is also relevant to rotary tuning.
00:30 Rotary engines are very sensitive to knock or detonation, though, and can quickly be destroyed if almost any level of knock occurs.
00:40 For this reason, it's important to tread carefully when tuning a rotary engine and be conservative with the amount of ignition advance being used.
00:49 It's always better to potentially leave a little power on the table, and have an engine that's going to last a long and healthy life.
00:59 Differences also exist in the sort of air-fuel ratios that we're likely to use in a rotary engine when compared to a piston engine.
01:08 The rotary engine shows a little less efficiency when compared to a piston engine, and this inefficiency shows up in the form of additional heat.
01:18 To help combat heat in the combustion chamber, we'll generally need to run a rotary engine a little richer than a comparable piston engine.
01:28 Two other important differences that you'll see on a rotary engine, are two spark plugs and two injectors per rotor, and these need to be dealt with by the engine management too.
01:41 The two spark plugs are referred to as leading and trailing, and they'll normally be fired at slightly different times in the engine's cycle.
01:50 When we talk about ignition timing in a rotary engine, we're normally referring to the ignition advance angle for the leading spark plug.
01:58 You can think of this as the main spark plug, and this is responsible for initiating the combustion.
02:05 The trailing plug is controlled seperately and will fire a little later after the leading spark plug.
02:12 This is referred to as trailing split.
02:15 The design of the rotary engine means that each rotor provides one combustion event for each rotation of the eccentric shaft, compared to a four-stroke piston engine, where we have one combustion event for every two rotations of the crank shaft.
02:31 It might seem like subtle difference, but in this detail lies one of the keys to the rotary engine making so much power out of a seemingly small capacity.
02:42 It also affects the fuel system requirements, though.
02:46 Or specifically what we're talking about here, the fuel injector requirements.
02:50 Since the rotary engine has a combustion happening twice as often as a piston engine, there's only half of the time available for the injector to deliver the fuel to the engine.
03:03 Coupled with this, a 13B for example, only has two rotors, so if that two-rotor engine is producing the same power as a four-cylinder engine, each rotor is essentially producing twice the power of each cylinder on the piston engine, and here's the fuel requirements increase further.
03:24 This is why we usually see two injectors per rotor.
03:28 This is called staged injection, and allows a smaller, primary injector to be used when fuel demands aren't that high.
03:36 As engine RPM and power increase, so does the fuel demand, and the secondary injectors begin adding fuel too, in order to keep up with engine's demands.
03:48 While it may be possible to supply all the engine's fuel requirements using a single injector, that injector would need to be very large, and this can make it hard to control it idle and cruise.