Have you ever imagined the secret behind the power why with just a little quantity of fuel(gasoline or diesel), we can pile up fourteen people in a 10,000 pound vehicle and accelerate this vehicle up a mountain side at 70 miles per hour by just depressing a pedal? Today for few moments, I will like you to consider the same question.
Well, the secret behind this power lies within robots called Internal combustion engines(ICE). Did I just say robots? Yes robots! Because that’s what they are. As an engineering graduate, I am awed by the power of internal combustion engines and the complexity of their designs and variety of their uses.
In this article I will be explaining the working principle, troubleshooting techniques and maintenance procedure of an ICE.This article will come in series and will utilize the simplest designs of ICE for easy understanding. I will first explain the general principle of operation of the ICE, then I will be explaining each component of the engine. Finally we will be looking at problems associated with ICE, ways to troubleshoot these problems and how to properly maintain the engine in order to prevent these problems.
Firstly, I will like us to understand what an ICE is before we explore the mode of its operation.
WHAT IS AN ICE?
The acronym ICE stands for internal combustion engine. An internal combustion engine is an engine which generates torque using the products of combustion as its working fluid. It is in essence a heat engine that converts heat energy of a fuel into mechanical energy made available through a rotating shaft.
In practice, It is not the fuel that necessarily drives an IC engine but the chemical energy of the fuel derived from the combustion of the fuel. In internal combustion engines, the ignition and combustion takes place inside the engine in a special region within the engine called the combustion chamber .
Internal combustion engines consists of a fixed cylinder and a moving piston pinned to a connecting rod which is mounted on a rotating shaft.
There are generally two types of IC engines currently in production: spark ignition gasoline engines and the compression ignition diesel engines. Each of these engines can either be a two stroke or a four stroke engine. For the purpose of easy understanding I will focus on spark ignition engines.
HOW DOES AN ICE WORK?
An IC engine works basically on the principle of reciprocating piston movements within the cylinder. These reciprocating piston movements within the cylinder are called strokes. This is why IC engines are commonly called reciprocating engines.
A stroke characterizes a unique stage in the operation cycle of an IC engine. There are essentially four strokes in an IC engine: intake, compression, power and exhaust strokes. Based on design, IC engines are classified as two Stoke and four stroke engines. However, the most commonly used design of IC engines are four strokes.
Consider the image below. The image represents the working cycle of a four stroke engine and will form the basics of my explanation.
A four stroke engine requires four piston strokes or movements to complete a cycle. These strokes are represented on the image by the numbers 1,2,3 and 4. Having represented these strokes in numbers, let us consider them in details.
Intake stroke:
This is the first stage in the working cycle of IC engines as shown in the image above. An intake stroke is simply the downward movement of the piston from the top dead center(TDC) to the bottom dead center(BDC) of the engine.
The top dead center is the maximum point a piston can reach as it moves upward within the cylinder before it reverses and start moving downward away from the top of the cylinder. While the bottom dead center is the minimum point a piston can reach as it moves downwards before it reverses and start moving upwards.
During the intake stroke, the intake valves opens, allowing inflow of fuel and air. As the fuel and air mixture flows into the cylinder, the piston moves downward drawing in the fuel air mixture until it reaches the bottom dead center.
Compression stroke:
The compression stroke begins immediately after the intake stroke and is represented by the number 2, on the image above. In this stroke, the piston moves upwards Pushing the fuel air mixture back to the top of the cylinder. During this process, both the intake and exhaust valves are closed.
As the piston pushes the volume of the fuel-air mixture, the volume reduces until it gets into the clearance volume where it is compressed. Clearance volume is the volume of the fuel-air mixture between the cylinder head or cover and the piston head when the piston is at the top dead center(TDC).
In other words, clearance volume is the limit to which the piston head can push the fuel air mixture before ignition takes place. In the clearance volume, the fuel-air mixture is trapped and tries to escape but it cannot since neither the inlet nor exhaust valves are open. This creates pressure on the fuel-air mixture thereby increasing its temperature.
In essence the piston does work on the fuel-air mixture by compressing it. The high temperature and high pressure fuel-air mixture in the control volume gets ignited by the spark plug. The mixture explodes and burn, releasing heat and carbon(iv) oxide within the combustion chamber.
Power stroke:
In this stage, the hot expanding gases released during combustion forces the piston downward from the top dead center(TDC) back to the bottom dead center(BDC). Hence transferring torque to the crankshaft through the piston. This downward movement of the piston from the top dead center(TDC) back to the bottom dead center(BDC) is called the power stroke.
It is only in this stroke that work is done on the engine by the hot expanding gases(combustion products). This is why this stroke is also referred to as the working stroke. This stroke is represented by the number 3 on the image.
Exhaust stroke:
This stroke is the fourth stroke in an IC engine as shown in the simulated image above. This stroke marks the end of a cycle In a four stroke engine. In this stroke, the piston moves upward from the bottom dead center(BDC) Pushing the gases remaining in the cylinder after combustion upward back to the combustion chamber. As the piston moves closer to the top dead center(TDC), the exhaust valve opens to allow the passage of these gases away from the cylinder. The purpose of the exhaust stroke is to Clear the cylinder of burnt gases, in preparation for another intake stroke.
Each of these four strokes in an IC engine occurs at about 20millieconds causing the crankshaft to rotate at an average speed of 4500 revolutions per minute(rpm) for passenger automobiles and 1500rpm for diesel generators.
Now we know how these wonderful robots called internal combustion engines work. But think of it; if work is done on the engine only in the power stroke, what then provides the torque that pushes the piston downwards during the intake stroke and pushes the piston upward during the exhaust stroke? This question will be answered in the next article.
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