Internal Combustion Engine Performance and Characteristics

Item 4: Characteristics of the Internal Combustion Engine

1. Engine Performance

Chemical Energy (Fuel) / Thermal Energy (Combustion) / Mechanical Energy (Piston Displacement)

The chemical energy contained in the fuel is converted into heat through combustion. The resulting increase in pressure moves the piston, thus obtaining mechanical energy. Efficiency is calculated as follows:

n = (Energy obtained / Energy supplied) * 100 (%)

Power Loss:

• Heat Loss: Due to the cooling system and heat transfer to the outside.
• Mechanical Losses: Friction between moving parts and driving auxiliary devices.
• Chemical Losses: Due to incomplete combustion.

2. Types of Performance

• Thermal
• Mechanical
• Effective
• Volumetric

3. Key Features of Engines

• Engine Torque
• Specific Power
• Specific Fuel Consumption

3.1. Torque

Torque, also known as the turning moment, is the result of a force applied to a lever arm. It depends on the force (F) applied to the connecting rod and the length of the crankshaft elbow (d).

3.2. Specific Fuel Consumption

This relates the mass of fuel consumed to the power delivered. It depends on:

• Thermal Performance: Increases with compression, as higher temperatures and pressures are achieved. This is why diesel engines consume less fuel.
• Volumetric Performance: Worsens with an increasing system, leading to increased fuel consumption.

4. Characteristic Curves

4.1. Power Curve

This curve shows the power values at different engine speeds (RPM). It is expressed in kW or hp.

Interpretation:

• Steep Slope: A small increase in RPM results in a significant increase in power. When the engine is operating near its maximum power, the engine speed will rise easily. However, recovering from low RPM will be difficult due to the significant energy required to overcome resistance. A lower gear should be used in this case. A steep curve indicates high specific power but low elasticity.
• Shallow Slope: A small increase in RPM results in a moderate increase in power. This makes it easier to recover from low RPM, as less effort is needed compared to the previous case. This type of curve is flatter, indicating greater engine elasticity, although the peak power figures are lower. To increase power, engine capacity is often increased.

4.2. Engine Torque Curve

This curve represents the evolution of torque as a function of engine speed. It is expressed in Nm (SI) or mkg (conventional). A flatter torque curve indicates greater elasticity. The torque curve generally follows the power curve.

4.3. Specific Consumption Curve

This curve represents fuel consumption with respect to the number of revolutions. It is measured in g/kW*h.