w2s1-engine basic concepts

8/17/2019 W2S1-Engine Basic Concepts

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© IFPEN / IFP School 2014

Sustainable MobilityTechnical and environmental challenges for the automotive sector

Week 2 – Session 1 – Engine basic concepts

Maria Thirouard


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W2 – S1 – Engine basic concepts p. 1


What is an engine?

In this chapter we will take a look at some basic knowledge about engines as you will need to be

familiar with certain concepts regarding engines to better understand the fuel properties and the

refining processes.

To answer this first question we can say that

an engine is a machine designed to convert

energy (in our case, from the fuel) into useful

mechanical motion.

Even though there is a wide variety of types,

the engines used in cars, buses, trucks and

tractors are all very similar and share many

common types of components. We need

everyone to be familiar with the main enginecomponents before we explain how an engine

works so we’ll go through a brief description

of those components as part of a terminology

exercise.

Engine: basic parts

You see, when the piston is in the top position, we call it the top dead center. In the opposite way,

when the piston is in the bottom position, we call it the bottom dead center.


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The distance between the top dead center or TDC and the bottom dead center or BDC, which is

the distance the piston actually moves each time, is called the stroke.

The inside diameter of the cylinder is called the bore. The dimensions of the stroke and the bore

give us the total volume swept by the piston in one stroke, what we call the volume displacement

of an engine.

So when we say that an engine has 4 cylinders and it’s a 2 liter engine, it means that the total

volume swept by the piston is 2 liters. By using a very simple equation, the total volume

displacement is calculated as the number of cylinders, times the stroke, times π, times the boresquared, divided by 4.

The last definition in this terminology section is the compression ratio. The compression ratio is the

proportion of the volume inside the cylinder when the piston is at BDC over the volume inside thecylinder when the piston is at TDC.


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That’s it for the terminology part. Of course this is a very simplified description but the main parts

are there and we will need this in order to understand how an engine works.

Now, engines can be classified in many different ways: by the engine cycle used, the layout, the

energy source, the configuration etc... Most of the engines used as a power supply for cars follow a

thermodynamic cycle with four basic steps that are constantly repeated every two revolutions: they

are called 4-Stroke engines.

Stroke Engines: General Principle

The first stroke is the intake stroke. This is also known as a suction stroke because the piston

moves to the Bottom Dead Center position creating a drop in pressure. The inlet valve opens and

the vaporized fuel/air mixture is sucked into the combustion chamber. The inlet valve closes at the

end of this stroke


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The compression stroke is next. Both valves (intake and exhaust) are closed and the piston starts

moving up towards the Top Dead Center position. The fuel/air mixture is compressed and as a

result, pressure increases inside the cylinder, as can be seen on the graph.

The third step is the power step. When the piston reaches a point just before the Top Dead Center,

the spark plug ignites the fuel mixture. The exact point at which the fuel burns might vary but the

pressure increase produced by combustion forces the piston back down towards the Bottom Dead

Center. The power produced by gases pushing the piston is transmitted to the crankshaft via the

connecting rod mechanism.


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Finally, at the end of the power stroke, once the piston reaches the Bottom Dead Center position,

the exhaust valve opens. The piston starts moving up again, pressure inside the cylinder drops and

the open exhaust valve allows the gases to escape from the cylinder. This is called the exhaust

stroke.

At the end of this stroke, the exhaust valve closes, the inlet valve opens again and the cycle is

repeated all over again.

As you can see, 4-stroke engines require two revolutions to complete a full engine cycle.

Gasoline and Diesel engines

Now that we’ve seen the working principle of a 4-stroke engine, let’s move on to the differences

between gasoline and diesel engines. Gasoline and diesel are both petroleum products but their

properties are different. Consequently, engines run on diesel or on gasoline have some different

characteristics.

Gasoline engines

We’ll start by the gasoline engine since gasoline is the fuel used worldwide for passenger cars.

Gasoline engines are often called spark-ignition engines. In these engines a mixture of fuel and air is

usually pre-mixed in the intake system before entering the combustion chamber.

Since the air/fuel mixture is done before entering the combustion chamber, the mixture is very

homogeneous. A spark plug is then used to time the combustion for optimal performance.

Indeed, it is the spark that initiates combustion so that the homogeneous mixture will burn.


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Diesel engines

Even though it is common in Europe to have a passenger car running with a Diesel engine, diesel is

most commonly known around the world as the engine for heavy duty applications.

Diesel engines are often called compression ignition engines and the process differs from the

gasoline counterparts in several significant ways.

In Diesel engines, air is brought into the combustion chamber through the intake system, often at

boosted pressure thanks to turbocharging. As the piston rises, the air is compressed to a much

higher pressur than in gasoline engines thanks to a much higher compression ratio of the Diesel

engine.

To properly time the start of combustion in a Diesel engine we rely on the injection of the fuel

directly into the combustion chamber that now already contains hot, high pressure air.

Instead of a spark, it is the dynamics of highly compressed air that is sufficient to initiate

combustion when the fuel is spread into the cylinder. However, in reality, the combustion does notstart instantaneously. There is a short delay between the initial injection of fuel and the start of

combustion. During this delay, Diesel is being vaporized and mixed to the air. When the combustion

starts, the mixture is still on going and is essentially heterogeneous.

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