“Simply Put” 4-Stroke Gasoline Engines

by Dan Rich, Contributing Editor COPYRIGHT © 2004

Modern Gasoline Engines

Let’s talk about your engine. Not specifically your engine, but generally all modern gasoline engines. The concepts presented here apply to virtually all engines in any modern SUV or truck.

The purpose of the engine is to provide power to move your car down the road. In order to do this, it turns gasoline into energy that can be used to move that machine. The gasoline engine has been around for over a century and has made significant progress, especially in the last 10 to 20 years, and has evolved into what we have today.

With exhaust emission and air quality regulations imposed on car manufacturers, and the cost of fuel continuing to go up, manufacturers have made great strides making your engine more powerful and efficient. One might be able to argue that without government intervention, this evolution would not have been so rapid, but that’s a discussion for another day.

4-stroke engine

The engine in your car is technically known as an internal combustion, spark ignition, 4-stroke engine. Internal combustion because the combustion event happens inside the cylinders, and 4-stroke because it takes four strokes of the piston to accomplish one “Power” stroke. These four strokes require 2 revolutions of the crankshaft, which the pistons are attached to. A 4-stroke engine can be anything from one single cylinder to a V-8 or V-12 engine. Don’t confuse 4-stroke with 4-cylinder. There’s a good chance both your lawnmower and your car have 4-stroke engines. We’ll talk about number of cylinders later.

The four strokes of the piston are: Intake, Compression, Power, and Exhaust, sometimes known to gearheads as “Suck, Squeeze, Pop, Blow.”

Order of events – Intake Stroke:

– – Piston near Top Dead Center (TDC)

– – Intake valve opens, allows air/fuel mixture into cylinder

– – Piston travels down the cylinder bore, creates negative pressure

– – (Fuel injector or carburetor supplies fuel to the cylinder)

– – At Bottom Dead Center (BDC), one stroke completed

Intake Stroke: The piston starts at the top of its stroke in the cylinder bore. As the piston travels down it creates negative pressure in the cylinder. The intake valve opens allowing a very carefully metered mixture of fuel and air that fills the cylinder.

Order of events – Compression Stroke:

– -Piston passes Bottom Dead Center (BDC)

– -Intake and exhaust valves closed.

– -Piston travels up the cylinder bore, creates high pressure and heat.

– -Air/fuel mix is compressed, waiting for the ignition source from the spark plug.

– -At Top Dead Center (TDC), second stroke completed.

Compression Stroke: This starts next when the piston begins its upward travel. The intake and exhaust valves are closed and the piston compresses the trapped air/fuel mixture causing a rapid temperature rise. When the piston is near the top of this cycle, the spark plug is fired.

Order of events – Power Stroke:

– Piston reaches close to Top Dead Center (TDC)
– Intake and exhaust valves closed.
– Spark plug fires, ignites the air/fuel mixture
– Piston is FORCED down the cylinder bore due to the explosive energy in the air/fuel mixture.
– This is the only stroke to contribute power to the vehicle.
– At Bottom Dead Center (BDC), third stroke completed.
Power: The combination of intense heat combined with spark plug ignition causes a controlled explosion within the cylinder, expanding the gases and forcing the piston down rapidly. How well your engine does this, combined with the size of the cylinder and piston (cylinder volume) determines how much power your engine creates.

Exhaust: With all of the available energy extracted from the air/fuel mixture, the piston now starts its way up the cylinder for the fourth stroke. The exhaust valve opens and the spent gases are now forced out of the cylinder and through the exhaust system of the vehicle. Once finished, the very next stroke, or down-travel, of the piston is the succeeding intake stroke.

Turning it Into Power: As the pistons do their thing, harnessing this converted power is the job of the crankshaft. Each piston is attached to the crankshaft using a connecting rod to turn the reciprocating (up and down) motion of the piston into rotary motion (‘round and ‘round) of the crankshaft and ultimately the flywheel where the transmission is attached.
Overhead valve or overhead cam

Aren’t they the same thing? Simply put, no. But they do accomplish the same thing, that is open the valves to let both the air/fuel mixture in and the spent gases out of the cylinder.

Overhead valve (OHV)

More than describing the location of the valve, this tells a knowledgeable person the location of the camshaft. Confusing? Well, it really means that ONLY the valves are located in, or ‘over’ the ‘head’ of the engine. The head is the part that goes on top of the piston and combustion chamber. In an OHV arrangement, the valves are above the piston and driven by pushrods and lifters. The camshaft drives the lifters, which are both located in the engine block. The camshaft and crankshaft are connected together using a chain and gears. For every two revolutions of the crankshaft, the camshaft turns one time. In other words, the gear on the camshaft is exactly twice as big as the gear on the crankshaft.

Overhead Cam (OHC, SOHC or DOHC)

In this type of engine, not only are the valves located in the head, the cam is also there, and the valves are driven directly off of the cam. The cam is still connected to the crankshaft in a 2:1 ratio, but this time the belt or chain attaches to the end of the cam all the way on top of the engine. It’s common to have either one or two camshafts to drive the valves in and overhead arrangement. This is called SOHC (Single OverHead Cam) and DOHC (Double OverHead Cam). If the engine is an in-line arrangement such as a four cylinder, there are one (SOHC) or two (DOHC) cams. If the engine is a V arrangement (V6, V8, V12), there are one or two cams per bank.

Which is better? There are plenty of people in both camps to argue the efficiencies and attributes of either. Both OHV and OHC are used commonly today, but the trend is definitely shifting towards the use of OHC due to its adaptability and capability to meet fuel economy and emissions demands set by auto manufacturers.

Getting fuel to the cylinder.

There are currently two basic ways to meter the fuel in order to get it into the cylinder – Carburetion and Fuel Injection. Ideally, the gas engine requires a very specific combination of air and fuel to create an optimal balance of power and fuel economy. That mixture is 14.7:1. That is, 14.7 pounds of air to 1 pound of fuel. Considering a gallon of gas weighs slightly less than 8 pounds, that’s a lot of air! For every gallon of gas, you use about 115 pounds of air. Keep your air filter clean!


This is the original method to manage fuel delivery to the engine. In a carbureted system, fuel is pumped at low pressure up to the carburetor on top of the engine. As air is drawn through the carburetor due to the pumping action of the piston, fuel is drawn out of small, metered holes and tubes within the carburetor and mixed with the air from that point on. The carburetor relies on vacuum and gravity to do its job effectively. In a 4-wheel drive vehicle, gravity isn’t always pulling in a direction that’s ideal for the carburetor. On a steep slope for example, the fuel pressure to the carburetor may be affected, or the fuel level changes, and the carburetor will “lean out” and have too little fuel available to do its job. Another downside is that on rocky, bumpy roads, which are the most fun to drive, the carb can find itself splashing its fuel around also causing a lean or even rich condition. Lean means to not have enough fuel, rich is just the opposite, or too much fuel and is characterized by black smoke out the tail pipe. The upside of carburetors is that they are inexpensive, simple and easy to fix on the trail or in the garage by a reasonably talented do-it-yourselfer.

Squirt, squirt – Fuel Injection

Some form of fuel injection is currently the only method that is used by auto manufacturers for fuel metering in modern gas, or even diesel engine. Today’s fuel injectors are electronically controlled “valves” that inject a very precise amount of fuel at precisely the correct time in the combustion cycle. Other sensors from the engine determine how much fuel is needed and then the signal is sent for the injector to open for a certain time allowing fuel into the combustion process. The injector is usually placed right before the intake valve allowing a precise blending with the air as it goes through the intake valve. By this method if everything is working well, the engine consumes only the fuel it needs to make the power it was intended to use. The down side to injectors is their seeming complexity and electronic nature attached as they are to a computer. They can also clog easily if good gas is not used, and do tend to clog or wear over time. The upside is that the engine will run well over bumpy roads or up steep grades without the same ills that the carburetor creates.

This article covered the basics of how most automotive engines used today turn gasoline into the power you need. Check out the other “Simply Put” articles on www.bb4wa.com for additional information about your vehicle.

COPYRIGHT© Dan Rich, 2004.
Material in this article may not be reproduced in any fashion without the express written consent of the author.