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Variable Cam Control Tuning: Atkinson and Miller Cycle

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Atkinson and Miller Cycle

02.54

00:00 - One of the advanced abilities that's possible on some engines equipped with cam control is to replicate the atkinson cycle.
00:07 The atkinson cycle engine was developed back in the late 1800s with the intention of improving efficiency albeit at the expense of power density.
00:17 WIthout getting too deep into the specifics, an engine that provides a longer expansion cycle or power stroke when compared to the compression stroke, can allow more of the energy from the fuel to be recovered.
00:30 The goal with an atkinson cycle engine is to allow the combustion gases to expand during the power stroke to the point that the pressure in the cylinder is at or very close to atmospheric pressure.
00:42 If this can be achieved then all of the available energy from the combustion process has been captured in order to provide torque at the crankshaft.
00:50 The downside with the atkinson cycle engine is that the shorter inlet stroke results in a reduction in the engine's volumetric efficiency.
00:59 While it's true that the engine will extract more energy from the same quantity of fuel and air compared to a conventional otto cycle engine, the problem is that we're sacrificing the amount of air and fuel that makes its way into the cylinder in the first place.
01:14 The original atkinson cycle engine design achieved this mechanically with a relatively complex system of mechanical linkages and arms to create the required variation in the cycles which wouldn't be workable on our modern high RPM high power engines.
01:31 We can however achieve the same effect by purposefully keeping the inlet valves open very late into the compression stroke.
01:39 We've already discussed that even in a conventional engine there's a cylinder filling advantage in keeping the inlet valves open some way into the compression stroke so that we can take advantage of the ram effect from the air velocity in the inlet ports.
01:55 By continuing to hold the valves open later in the cycle, the piston will actually force some of the fresh inlet charge back out of the cylinder and into the inlet manifold.
02:04 This has the effect of reducing the length of the compression stroke since for a portion of the compression stroke, the inlet charge is freely escaping back past the inlet valves instead of being compressed.
02:15 The power stroke on the other hand is unchanged.
02:18 As mentioned, the downside is that the engine's volumetric efficiency and hence power density is reduced when running the Atkinson cycle.
02:26 This can be addressed with the addition of forced induction and in this instance, it's referred to as a miller cycle.