Diesel engines don’t have a throttle body because of the fundamental differences in how they operate compared to gasoline engines. This causes them to have much less engine braking. To help a large truck slow down without overheating the service brakes, a Jake brake, which is a compression release engine brake, is used. The name comes from Jacobs Vehicle Systems that first made them in the 1960s.
Fundamental differences in operation of petrol vs diesel engines
- Air-fuel mixture control:
- Petrol engines: In a petrol engine, the air-fuel mixture is controlled by the throttle body, which regulates the amount of air entering the engine. The fuel is mixed with this air before it enters the combustion chamber. The throttle body’s position, controlled by the accelerator pedal, determines the engine’s power output by varying the air intake.
- Diesel engines: Diesel engines operate on the principle of compression ignition, where only air is compressed in the cylinder, and fuel is directly injected into the compressed air. The power output is controlled by varying the amount of fuel injected rather than controlling the air intake. This eliminates the need for a throttle body to regulate airflow.
- Combustion process:
- Petrol engines: These engines use spark plugs to ignite a pre-mixed air-fuel mixture. The mixture ratio must be controlled within a certain range to ensure efficient combustion and prevent knocking, hence the need for precise air control via the throttle body.
- Diesel engines: Diesel engines ignite the fuel by the heat of compressed air. The amount of air entering the engine is relatively constant, and power is managed by adjusting the fuel injected into this air. Since the air is not mixed with fuel before entering the cylinder, there’s no need to throttle the air.
- Efficiency and design:
- Petrol engines: Throttling the air controls engine speed and power but creates pumping losses, which reduce efficiency.
- Diesel engines: By not throttling the air, diesel engines avoid these pumping losses, contributing to their greater efficiency and better fuel economy compared to petrol engines.
Components specific to diesel Engines
Instead of a throttle body, diesel engines use other components to control power and performance:
- Fuel injection system: This is the primary means of controlling engine power. Modern diesel engines use high-pressure fuel injectors that can precisely control the amount and timing of fuel injected into the combustion chamber.
- Turbochargers and intercoolers: Many diesel engines use turbochargers to increase the amount of air entering the cylinders, improving efficiency and power. Intercoolers cool the compressed air from the turbocharger, making it denser and increasing the oxygen available for combustion.
- Exhaust Gas Recirculation (EGR) and Variable Geometry Turbochargers (VGT): These control emissions and improve efficiency by managing airflow and exhaust gases more effectively.
How important is engine braking in lorries?
- Commercial semitrailers can weigh up to 42,500kg, and road trains can weigh up to 200,000kg, and that requires additional braking support.
- Continuous use of brakes on long descents can cause overheating and failure (brake fade).
- The Jake brake provides essential engine braking for these trucks.
Mechanics of a Jake Brake:
- The Jake brake, or compression release brake, uses the engine’s compression to slow down the vehicle.
- During the compression stage, air is compressed and pushes back against the piston.
- Instead of allowing the air to push the piston down during combustion, the Jake brake releases this air, slowing the engine.
Comparison to exhaust brake:
- Exhaust brakes create back pressure to slow the engine, using an exhaust flap or variable geometry turbocharger.
- Jake brakes use engine compression, making them fundamentally different from exhaust brakes.
Applications and legalities:
- Jake brakes are crucial for commercial trucks but unnecessary for smaller diesel cars.
- The noise produced by Jake brakes can be regulated and restricted in urban areas.
Benefits and limitations:
- Provides safer vehicle operation and reduces brake wear, saving costs.
- Limited braking force distribution can be problematic in conditions like snow.
- Newer trucks have quieter Jake brakes, but older models can be very loud.
Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists
