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Expert mixologists at work

We know now that your engine generates motion from heat energy.

india Updated: Aug 27, 2009 10:58 IST
Grease Monkey

We know now that your engine generates motion from heat energy. We also know that this heat energy is obtained by burning fuel and air in a particular proportion to extract maximum efficiency from the fuel. This time, we’ll take a closer look at what keeps an eye on the amount of air and fuel that goes into the engine for combustion.

Carburettors have handled the job of mixing fuel and air since the beginning of the automobile, and they are a purely mechanical way of doing things. Karl Benz (who lends his name to ‘Mercedes-Benz’) invented the ‘carb’ a little before 1885. Most Indian bikes still use carburettors since they are inexpensive, but almost all contemporary cars have moved on to fuel injection.

A carburettor has different ‘circuits’ that supply fuel depending on the requirements. An important part of the carburettor is the throttle body, which is a plate in the shape of the intake pipe that can rotate along the diameter of the pipe. When it is perpendicular to the pipe, it closes off airflow, and when parallel, it offers zero resistance.

This is what makes the engine revs rise and fall in a carburetted engine. A bigger throttle body can help supply more air to the engine, so it can be used to improve an engine’s performance. If you’ve ever heard someone talk about ‘downdraft’ or ‘sidedraft’ carbs, it refers to the direction of airflow in the carb: the air flows downward in a downdraft carburettor. High temperatures can affect a carburettor’s performance, as can substantial changes in altitude.

As with things like bank tellers, typewriters and 8-bit video games, computers are now replacing carburettors as well. Fuel injection was first used in a production car on the Mercedes-Benz 300 SL, which is known more for its gullwing doors. A fuel injection (FI) system consists of a fuel pump, injectors and sensors. The ECU (engine control unit) is the brain. It takes its information from the sensors and decides how much fuel has to go into the engine. The fuel pump sucks up fuel from the tank and sends it to the injectors under pressure. If you look at the warning lights on your dashboard while switching your engine on, you will notice it is one of the lights that goes off after a while, just like the ABS, airbags and traction control lights.

The injectors squirt minute amounts of fuel at a time, contributing to the air-fuel mixture. Except think of them more as spraying the fuel than injecting it, because that helps the fuel mix better with air. Sensors that measure everything from throttle position to the amount of oxygen in the exhaust contribute to the ECU’s decision on how much fuel goes in, as well.

The ECU decides on the basis of a table it has that goes something like this — ‘if x amount of throttle is applied then squirt y amount of fuel into the mixture’.

There has been a revolution recently — diesels have begun using common-rail direct injection. Even petrol engines are undergoing a slow but steady shift to direct fuel injection because of the lower emissions and higher fuel efficiency figures it offers. You may have heard of FSI or GDI or DFI, which are all terms for direct injection in petrol engines. We’ll have a look at these in the future.

If you have questions or comments for Grease Monkey,
email him at with Auto Tech 101 in the subject line.