Piston Engines - Carburettors — Page 100, Lesson 127

All right, let's get into the carburettor. This is the heart of how a piston engine breathes and drinks, and we're going to look at the basic requirements first. The job of the entire carburation system is to control the air/fuel ratio. That's the proportion of air to fuel being mixed. And it has to do this in response to the throttle setting, at all selected power outputs, from slow-running right up to full throttle, and it has to do it smoothly during acceleration and deceleration. So it's not just about a static setting; it's about the transition between them. It also must function at all altitudes and temperatures in the operating range. So whatever the conditions, the mixture has to stay correct. And finally, it must provide for ease of starting, and it may incorporate a means of shutting off the fuel to stop the engine. That's your fuel shut-off. Now, there are three main ways to achieve this. The first is the float chamber carburettor. This is the cheapest and simplest arrangement, and it's used on many light aircraft. But it has two significant weaknesses: it is very prone to carburettor icing, and it may be affected by flight manoeuvres. Think of the float sloshing around in a steep turn. The second is the injection carburettor. This is a more sophisticated device. It meters fuel more precisely, which gives a more accurate air/fuel ratio. It's also less affected by flight manoeuvres and less prone to icing. So it's a step up in both precision and reliability. The third is the direct injection system. This provides the best fuel distribution and is reputed to be the most economical. It is unaffected by flight manoeuvres and is relatively free from icing. So each system is a progression in terms of precision and immunity to the problems of the float chamber type. Now, any of these systems may be fitted with a manual mixture control. That's a lever in the cockpit that lets the pilot lean the mixture out for the most economical cruising mixture. But to help the pilot select the best mixture, some aircraft are fitted with fuel flowmeters or pressure gauges, or exhaust gas temperature gauges. Those are your instruments for fine-tuning the mixture in flight. One last point: diesel engines do not have carburettors. They do have an inlet system to allow air to be induced towards the cylinders, and that incorporates an air filter. But the key difference is that the air supply is not 'throttled'. So there's no throttle plate restricting the air intake on a diesel. Let me show you the general layout of a typical carburettor system so you can see how these parts fit together. So to summarise the three systems: float chamber is cheap and simple but prone to icing and manoeuvre effects; injection carburettor is more precise and less affected; direct injection is the best for distribution and economy. And remember, the mixture control and the associated gauges are there to help you manage the air/fuel ratio in cruise.