How Your Engine Works
Even though engines have become more complicated recently with computer controls and such, all internal combustion piston engines basically work the same way. Almost all car and truck engines are four cycle piston engines, commonly known as four stroke motors.
There are other types of engines: the Wankel is a rotary engine that Mazda still uses in the "RX" series, big truck diesels are often two stroke piston engines (diesels), and SAAB once made a car with a two stroke gas engine. This article is just about four stroke motors.
All internal combustion piston engines burn a fuel and air mixture inside a cylinder. A piston inside this cylinder is forced to the bottom of the cylinder by the pressure of the burning fuel. The piston is attached to a connecting rod, which turns a crankshaft.
A "stroke" is when the piston travels from one end of the cylinder to the other.
When the piston goes from the top of the cylinder to the bottom it is said to be "on the down stroke".
When it travels from the bottom of the cylinder to the top it is said to be "on the up stroke".
THE STROKES
Four cycle engines take four strokes: the intake stroke, compression stroke, power stroke, and the exhaust stroke, then repeat the cycle. The crankshaft rotates two times to complete the four strokes, two upstrokes and two downstrokes.
THE INTAKE STROKE
On the intake stroke the piston is on a down stroke. The intake valve is open. In a gas engine the piston draws in a fuel and air mixture to burn. On a diesel just air is drawn in: the fuel is injected directly in to the cylinder later.
THE COMPRESSION STROKE
The compression stroke is next: it is an upstroke. During the contents of the cylinder are compressed, creating pressures of 150 pounds or more on a gas engine or 300 pounds or more on a diesel. Under this pressure the cylinder contents ignite easily and burn almost completely.
THE POWER STROKE
The power stroke is a down stroke. The buring fuel forces the piston down and turns the crankshaft.
The compressed fuel air mix is ignited by a spark plug in a gas motor. A diesel is a "compression ignition" motor: It has injectors which spray fuel directly into the cylinder. Because of the highly compressed air and heat in the cylinder, the fuel spontaneously ignites without the need for a spark plug.
THE EXHAUST STROKE
The exhaust stroke is an up stroke. The exhaust valve opens and the piston moves to the top, forcing the burned exhaust gasses out of the cylinder.
VALVES AND CAMSHAFTS
The valves are opened by a camshaft. It has eccentric "lobes" which press against "valve lifters" which open the valves.
CYLINDER BLOCK AND CYLINDER HEAD
There are 2 major parts to your engine: the block and the head. The block contains the pistons and crankshaft. The head contains the valves. The camshaft can be either in the block or the head. If the cam is in the head, it is called an OVERHEAD CAM ENGINE. All four stroke engines have at least two valves per cylinder: an intake valve and an exhaust valve. Many engines now have 3 or more valves per cylinder, with 4 valves being common. With 3 valves they use 1 exhaust and 2 intakes. With 4 they use 2 intakes and 2 exhausts.This is done to increase flow through the valves of the fuel air mix and increase the power and efficiency of the engine. The downside is that it makes them more expensive to repair!
MANIFOLDS
An intake manifold connects to the head and admits the fuel air mix, and the exhaust manifold connects to the vehicle's exhaust system. There will be one "port" (tech for "hole") in each manifold for each cylinder.
In order for an engine to run properly the valves must open and close at exactly the right time. A proper fuel/air mixture must be in the cylinder. Engine timing is expressed in degrees of crankshaft rotation. When the piston is at the top of its upstroke, it is said to be " AT TOP DEAD CENTER (TDC)" or 'AT ZERO DEGREES". Most all gas engines fire their spark plugs before top dead center, so timing specs often say things like "4 degrees BTDC", or Before Top Dead Center
When the piston is at the bottom of the downstroke it is at "BOTTOM DEAD CENTER" or 180 degrees.
On a gas engine IGNITION TIMING is the time the spark plugs fire relative to the position of the piston, or crankshaft.
Diesel engines don't have spark plugs, but they have INJECTION TIMING: the fuel must be injected into the cylinder at the right time or a diesel won't run!
VALVE TIMING is the time the valves open relative to the piston, or crankshaft.
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Thank you for visiting the ECONOMECHANIX WEB SITE. Please feel free to comment. We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry! Gainesville has been my home since 1974, and I've loved Gvl and the Gators since I came here in the fall of 1974 to attend the University of Florida. I loved it so much I stayed and opened my car repair business. Originally it was out of the back of a 1963 Chevrolet wagon, but in 1977 a fellow mechanic and I opened an auto repair shop with actual walls, etc. I stayed in the same location for 26 years, and recently moved my operation to property I bought 15 miles east of Gainesville. I am doing most all the repairs myself now, having reduced my overhead from $1500 per month to practically nothing. I do work by appointment only. I mostly work only on my established customers cars, but I will occasionally take on new clients. E-mail me and I will either make arrangements to look at your car, or I will recommend you to someone who will.
George G. Scott, Jr.