Understanding Torque in SI Engines and Engine Speed

Torque is a rotational force that an engine produces. It represents the engine's ability to do work at a certain speed. In a Spark Ignition (SI) engine, the torque produced varies depending on several factors, including the engine speed.

How Engine Speed Affects Torque in SI Engines

When you look at a typical torque curve for an SI engine plotted against engine speed, you observe a characteristic shape. The torque does not remain constant as the engine speed increases. Instead, it follows a specific pattern:

• Low Engine Speeds: At very low speeds, the engine's ability to fill the cylinders efficiently (volumetric efficiency) might be limited. Also, combustion might not be perfectly optimized at the slowest speeds. This often results in relatively lower torque output.
• Increasing Engine Speeds: As the engine speed increases from low levels, the engine generally becomes more efficient at drawing in the air-fuel mixture. Combustion timing and efficiency also tend to improve up to a certain point. This leads to an increase in the pressure applied to the piston, resulting in higher torque being generated. Torque continues to increase as speed rises.
• Peak Torque: The torque reaches a maximum value at a specific engine speed. This speed is where the combination of volumetric efficiency, combustion effectiveness, and frictional losses results in the highest torque output.
• High Engine Speeds: Beyond the speed at which peak torque is achieved, several factors cause the torque to decrease. These include:
  • Increased friction within the engine.
  • Reduced volumetric efficiency due to flow restrictions in the intake and exhaust systems as the speed gets very high, leaving less time for the cylinders to fill/empty properly.
  • Incomplete combustion because there is less time available for the burning process to complete fully at extremely high speeds.

Considering these points, the torque produced by an SI engine typically starts relatively low at idle or very low speeds, increases as the engine speed rises, reaches a peak value, and then decreases as the speed continues to increase towards the maximum engine speed.

Therefore, as the engine speed increases, the torque first increases and then decreases.