Chapter 4 Engine terms

Bore

A cylinder bore is part of a piston engine. Also, a bore represents the diameter size of the cylinder in which the piston travels.

Indirect Injection (IDI)

A compression ignition (CI) or spark ignited (SI) engine where fuel is not directly injected into the combustion chamber, rather the fuel charge is introduced outside the engine cylinder. Gasoline engines are usually equipped with indirect injection systems, where a fuel injector delivers the fuel at some point before the intake valve.

Internal Combustion Engine

A heat engine that gets power from the combustion of fuel inside the engine cylinder. This type of engine differs from a steam engine that generates power from the combustion of fuel outside the engine.

Engine

A machine that converts one form of energy to another form of energy. In an engine, chemical energy in fuel is mixed with air and then inducted into the cylinder during the intake process. After the piston compresses the fuel-air mixture, a spark ignites it, causing combustion. The expansion of the combustion gases pushes the piston during the power stroke. The result is mechanical energy which accelerates the vehicle.

Compression Ratio

A measure of the cylinder volume when the piston is at the bottom of its stroke (BDC), and the cylinder volume when the piston is at the top of its stroke (TDC). Compression ratios in diesel engines are 14:1 - 24:1. Turbos and bus diesel engines have compression ratios of 16:1 - 17:1.

After Top Dead Center (ATDC)

A point of piston travel through its down-stroke

Ratio

A quantitative relationship between two amounts that shows the number of of times one value is contained within the other. Example the ratio of turbos to non-turbos is 8 to 1. Ratios are used in automotive technology to describe the relationships of gears, levers, and cylinder compression.

Heat Engine

A system (engine) that converts heat energy to mechanical energy which is used to accelerate the vehicle. Heat (thermal) energy is produced from burning combustible fuel into mechanical energy that accelerates the vehicle. A diesel is a heat engine that uses diesel fuel to function.

Naturally Aspirated (NA)

An engine in which air intake depends solely on atmospheric pressure and not on forced induction through a turbocharger. The down-stroke of the piston gets the air into the cylinder. Sports cars use naturally aspirated engines to avoid turbo lag.

Square Engine

An engine in which the diameter of the cylinder bore is equal to the piston stroke dimension.

Over-square Engine

An engine in which the diameter of the cylinder bore is larger than the piston stroke dimension. Most spark-ignited, gasoline fueled engines are over-square.

Under-square Engine

An engine in which the diameter of the cylinder bore is smaller than the piston stroke dimension. Most diesel engines are under-square

Diesel Engine

An internal combustion engine in which heat that is produced by the compression of air in the cylinder is used to ignite the diesel fuel.

Spark Ignited (SI)

An internal combustion engine where the combustion process of air-fuel mixture is ignited by a spark from a spark plug. .

Compression Ignition (CI)

Any diesel engine (internal combustion engine) in which fuel that's been injected into the combustion chamber is ignited by heat generated on the compression stroke.

Before Top Dead Center (BTDC)

Any point of piston travel through its upstroke

Direct Injection (DI)

Either a CI or and SI engine where fuel is directly injected into the combustion chamber instead of the intake port. This improves combustion efficiency, increases fuel economy and lowers emissions. Most diesel engines and gas-fueled spark ignited (SI) engines use DI

Bottom Dead Center (BDC)

In an engine, the position of a piston that is nearest to the crankshaft is the BDC

Top Dead Center (TDC)

In an engine, the position of a piston that is the farthest from the crankshaft is the TDC.

Volumetric Efficiency

Is the engines ability to put air into its cylinders. The greater the efficiency percentage, the more completely the engine fills the cylinder volume. It's the measure of an engines breathing efficiency. Volumetric efficiency is the maximum volume of air that each cylinder can ingest during the intake cycle equal to the swept volume of the cylinder.

Manifold Boost

The air pressure in the inlet manifold. The pressure is the force of air into the cylinders in a turbo-charged engine. Most truck and bus diesel engines are turbo boosted or turbo charged.

Stroke

The distance through which a piston travels from BDC to TDC. A stroke is the travel of the piston along the cylinder.....the distance from the crankshaft center-line to the throw center-line. A four-stroke engine is an internal combustion engine in which the piston completes four separate strokes while turning the crankshaft.

Kinetic Energy

The energy of motion. Energy that an object has because of its motion. The faster the engine is moving, the more kinetic energy is has. In an engine the chemical energy in fuel is transferred into kinetic energy (motion) in the engine and wheels.

Compression Pressure

The pressure developed in an engine at the end of the compression stroke without combustion of fuel. The range for compression pressure in CI engines is from 350 psi to 700 psi and +/- 600 psi in truck diesel engines. The higher the compression pressure, the more heat developed in the cylinder.

Combustion Pressure

The pressure in the engine cylinder during the 4 strokes of engine operation (1. intake, 2. compression, 3. combustion and expansion, and 4. exhaust). it's the peak pressure during the power stroke.

Clearance Volume

The remaining space (volume) between the end of the cylinder and the last point till the piston changes its direction. When the piston is at the top of its travel (TDC) there is some gap left between the end and the point where the piston changes its direction to go back. The gap is called clearance volume. head and the piston top when the piston is at the top dead center (TDC). It's the space of cylinder that is not swept by the piston.

Engine Displacement

The swept volume of all the pistons inside the cylinders of an engine in a single movement from top dead center (TDC) to bottom dead center (BDC). It is specified in cubic inches or cubic centimeters/liters. [displacement = bore x bore x stroke x 0.7854 x number of cylinders]

Fire Point

The temperature that a flammable liquid gives off enough vapor for continuous combustion to take place. Also know as ignition temperature.

Swept Volume

The total space (volume) covered (swept) by the piston inside the cylinder as it as it moves from BDC to TDC.

Cylinder Volume

The total volume (space) in a cylinder when the piston is at (BDC) bottom dead center. [Cylinder volume = swept volume + clearance volume]