Piston Engines - General — Page 29, Lesson 31

We're starting a new topic now: Piston Engines — General. And the first thing I want to walk you through is a concept called Volumetric Efficiency. Here's the professional definition. Volumetric Efficiency is the ratio of the weight of mixture actually induced into the cylinder, compared to the weight of mixture that would fill that same cylinder under normal temperatures and pressures. And we express that as a percentage. So the formula is: Volumetric Efficiency equals the weight of mixture actually induced, divided by the weight of mixture which could fill the cylinder at normal temperatures and pressures, all multiplied by 100%. Let me unpack what that means in plain language. The "mixture" is the fuel-air charge. "Induced" means drawn in during the intake stroke. So we're asking: how much charge did the engine actually manage to pull in, versus the theoretical maximum it could hold if conditions were perfect? That percentage tells us how well the engine is breathing. Now, what affects it? Two categories. First, design factors — specifically valve lead, valve lag, and valve overlap. Those are the timing relationships of when the intake and exhaust valves open and close relative to piston position. Second, operational variables — exhaust back pressure, resistance to flow through the intake path, and the force pushing the mixture into the cylinder. That last one is key. If the force pushing the mixture in is simply the difference between atmospheric pressure and the cylinder pressure during induction, then the engine is said to be Normally Aspirated. No artificial boost — just atmospheric pressure doing the work. And here's a number to remember: a normally aspirated engine will have a maximum volumetric efficiency of between 75% and 85%. That's the ceiling for a naturally breathing engine. So how do we improve it? One way is to increase the force pushing the mixture into the cylinder. That's called Supercharging, and it's covered later in these notes. For now, just know that supercharging is the method of forcing more charge in to raise volumetric efficiency and therefore power. Now let's move to the second major concept: Compression Ratio. The work done on the mixture by the piston during the compression stroke depends on two things — the weight of mixture induced, and the pressure it's raised to. And that pressure rise depends on the reduction in volume. So we need to define three volumes, and they're illustrated in Figure 2.10. First, Total Volume — that's the volume above the piston when the piston is at BDC, Bottom Dead Centre, the lowest point of its travel. Second, Swept Volume — that's the volume displaced by the piston during a single stroke. And the formula is: Swept Volume equals the cross-sectional area of the cylinder multiplied by the stroke. The stroke is the distance the piston travels. Third, Clearance Volume — that's the volume above the piston crown when the piston is at TDC, Top Dead Centre, the highest point. This clearance volume forms the combustion chamber. Now here's the relationship that ties them together: Total Volume equals Swept Volume plus Clearance Volume. And the Compression Ratio of the engine is the increase in pressure — it's the ratio of the total volume to the clearance volume. That's the measure of how much the charge gets squeezed during the compression stroke. So to summarise where we are: Volumetric Efficiency tells us how well the engine breathes, capped at 75–85% for a normally aspirated engine, and supercharging is the way to push past that. Compression Ratio tells us how much the charge is compressed, built from those three volumes — total, swept, and clearance.