Combustion Engine

This was a question to me when I was new in driving cars. It was a puzzle to me why the car speeds up when the accelerator is stepped on. I never thought then that engine combustion is the compression of the correct mixture of air and fuel inside an engine's cylinder and then ignited. It never came to me then that volume compressed would either speed up or decelerate a car. The same ways that I never imagined then those proportions of fuel and air play a big role in combustion. Well, that was in my younger days when I did not care about it, as long the car was running. My interest and passion for cars made me learn that there are two different fuels for two different combustion types. For gasoline engines it is Spark Combustion because the compressed air-fuel mixture is ignited by a spark plug. For diesel fueled engines it is Compression Combustion because the compressed air in the cylinder is ignited when diesel fuel is injected at almost the end of the compression stroke. The compression of ratio of diesel engines is higher that it compresses from 20 of air to just 1. For gasoline engines it is lower, around 10 of the air-fuel mixture compressed to 1 before it is ignited by the spark plug. Both engines have intake and exhaust valves on each cylinder. Many newer model vehicles have two each of the intake and exhaust valves per cylinder. Intake valves open to let air or air-fuel mixture be sucked into the cylinder by the piston. The exhaust valves open after combustion and the piston is on its upward stroke to push out the combustion residue. Both valves are closed during the compression stroke.

A question that would be asked is, how can the piston compress the air or air-fuel mixture without it escaping? The piston is constructed in such a way that there are rings fitted on its grooved sides. Usually there are three rings. Two would be oil rings that scrape the engine oil to prevent it from getting inside the cylinder. A third ring is called a compression ring that seals the surroundings of the piston so air or air-fuel mixture would not escape from the cylinder. The fuel that is mixed with the air is atomized or vaporized form coming from fine spray fuel injectors. On cars with carburetors, the fuel is vaporized or atomized when air is forced down the carburetor venturi and through the intake manifold before it reaches the intake valve. With that, there is no liquid fuel that enters the cylinder. That is what happens on every cylinder of the engine alternately. The engine has a four-stroke cycle namely:

1. Intake Stroke 2. Compression Stroke 3. Combustion/Power Stroke 4. Exhaust Stroke

The question on how come the car goes faster when you step on the accelerator was answered. When the accelerator is stepped on, a bigger volume of air or air-fuel mixture is introduced into the cylinders. Bigger volumes compressed will result in stronger and more powerful combustion that makes the piston travel faster downward after combustion. That's how cars go faster or slower, it depends on the volume of combustible mixture introduced to the cylinders. When the engine is running on operating temperature, the air-fuel mixture is around 14 parts of air to 1 part of fuel, by weight. The engine requires a richer mixture of around 4 parts of air to 1 part fuel when starting the car from cold. This is how chokes are used for older model engines to keep a rich mixture until the engine reaches operating temperature. Newer model cars have automatic chokes or sensors that detect operating temperature of the engine and self adjusts when needed.

In my analysis, combustion plays a big role in fuel economy. Usually, the ideal speed for maximum fuel economy is somewhere around 80 kph. Below or beyond that the engine struggles more than what is needed. That just means to say that the ideal power-to-weight ratio is achieved at that speed. In layman's terms, the forces involving intake, combustion and exhaust are balanced-off. What is being implied here, on the last note, is how engine combustion works for you with reference to fuel economy.