I have written a lot in the past about the engines in our vehicles. I have talked about the engine’s torque and horsepower ratings. I have even discussed in some detail the operation of the computerized engine control system that keeps the engine running smoothly. I don’t think, however, I have ever discussed the fundamental operation of the engine in great detail.

The engines in our vehicles are really nothing more than pumps. Our cars engines are not unlike the hand pump we use to spray our hair with each day. It is not unlike the vacuum cleaner we use to clean our floors. Nor is it unlike the pump that delivers water to our homes. Every pump works off the same basic principle of operation. A pump transfers some substance, under pressure, to some other point. In the case of the hand pump mentioned above, hairspray is transferred from the bottle to our hair.

Our automotive engines primarily transfer air, from the atmosphere, and fuel, from our tanks, into our engines’ combustion chambers and then exhaust gases out the tailpipes. Our automotive engines’ (pumps) rotation is sustained by using some of the power that is developed in the combustion process to sustain the engine’s pumping action.

In order for any pump to work efficiently, there must be a pressure differential developed by the pump. You may recall from previous articles that high pressure always moves to low pressure. It is this law of physics that a pump utilizes in order to function properly. The greater the difference in pressure, the greater the movement of whatever substance the pump was designed to transfer.

As I mentioned earlier, our automotive engines are designed to transfer air and fuel. It does so by creating very low pressure within the combustion chamber on the intake stroke. Atmospheric pressure is usually about 14.5 pounds per square inch, while the pressure within the combustion chamber during the intake stroke is under a very deep vacuum. This pressure differential causes the atmospheric air to force its way into the engine, through an air filter and into the low pressure combustion chamber. A predetermined amount of high-pressure fuel is sprayed into the air stream shortly before the combustion chamber.

Once the air and fuel have entered the chamber, the intake valve is closed and the piston squeezes the air and fuel into a very narrow space located between the top of the piston and the bottom of the cylinder head.

The compression of the air and fuel raises its pressure and temperature. At a point just before the air-fuel mixture would explode, the mixture is ignited by the spark plug. The air and fuel burns very rapidly and as it does, it expands exponentially. The oxygen within the air and all the fuel should have been consumed during this process.

This expansion of the burning air and fuel causes the piston to travel downward extremely rapidly. The force of the piston moving downward is transferred from the piston, via a connecting rod, to the engines crankshaft. This process is called the power stroke. The power created during this stroke is what causes our vehicles to function properly.

Once the pistons downward motion is reversed, by the rotational movement of the crankshaft, an exhaust valve will open and the burnt air and fuel will be forced out of the chamber under high pressure through the exhaust system and back into the atmosphere. Once the exhaust stroke is complete, the exhaust valve closes and the whole process starts again.

Now if your engine has multiple cylinders, as most do, then this process is occurring for each of the cylinders, sequentially, relatively independent of the rest. If any of the cylinders within an engine does not perform one of the tasks mentioned above efficiently, then the driver of the vehicle will notice an engine misfire, stumble or lack of power.

I prioritize the diagnostic sequence and processes for my students in order to effect the proper repair of a vehicle. My first rule of diagnosis is to verify the mechanical integrity of the engine. “Given a choice in diagnosis between a mechanical failure and an electrical failure, choose the mechanical failure and most often be correct” is how I advise my students. I advise you the same.

Any failure of the base engine, the pump, could result in multiple symptoms developing on a vehicle. The transmission may not shift correctly because of an engine problem. The heating and air conditioning system may not function properly because of a failure in the base engine operation. The on-board computer system may not function correctly or may trigger false codes as a result of a failure of the base engine. Many different symptoms could develop as a direct result of the engine’s integrity being compromised. Therefore, the function of the base engine operation must always be verified before attempting diagnosis on a system that depends on the engines correct operation.

E-mail columnist Nick Shultz at letters@toledofreepress.com.

This entry was posted on Friday, July 10th, 2009 at 12:00 am and is filed under Auto Sense, Wheels.
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