What Does Engine Braking Mean?
5 Answers
Engine braking refers to the braking effect on the drive wheels generated by lifting the accelerator pedal without depressing the clutch, utilizing the compression resistance, internal friction, and intake/exhaust resistance produced during the engine's compression stroke. Essentially, it means driving in gear without applying throttle, where the engine provides no traction to the vehicle. Instead, the rotation of the wheels drives the transmission system, and the idling engine generates a counteracting resistance on the vehicle. The higher the gear, the lesser the engine's effect on the vehicle, and vice versa. Engine braking is the process where the wheels overcome the engine's resistance to brake. Once the engine's passive speed reaches a certain threshold, it stops fuel injection, achieving fuel-saving purposes. Additionally, engine braking reduces wear on brake pads, extending their service life.
I love using engine braking when driving a manual transmission car on mountain roads. When going downhill for a long distance, I shift to a lower gear without stepping on the accelerator. The engine RPM will soar, but the speed is actually held back, which is especially good for saving brake pads. This essentially cuts off the fuel supply, causing the piston movement to be hindered, and utilizes the internal resistance of the engine to slow down. Veteran truck drivers call this 'gear braking,' which is much safer than just using the brakes, especially on rainy or snowy days when tire traction is poor. It’s both stable and prevents brake overheating and failure. However, be careful not to shift into too low a gear, as sudden traction force can cause the rear wheels to skid.
Just used engine braking while towing the RV last week. The moment I released the throttle, it felt like an invisible hand was pulling the vehicle, with speed decreasing remarkably linearly. The principle is simple: the transmission is connected to the wheels, and shifting to a lower gear essentially turns the engine into an air compressor. The repeated compression of intake air by the cylinders creates significant resistance, which is transmitted back through the drivetrain to slow the wheels. This effect is more pronounced in turbocharged vehicles—the whooshing sound of the blow-off valve is clearly audible. Compared to frequent brake application, this method better protects the braking system when hauling heavy loads.
Engine braking essentially utilizes the idle resistance of an internal combustion engine. Based on my actual measurements during car repairs: when coasting in 3rd gear, the engine's drag force is equivalent to 30% of braking force. Lower transmission gears provide greater resistance - for example, 2nd gear can achieve a deceleration G-force of -0.3G. However, automatic transmission vehicles require attention: when coasting in D gear, the transmission will upshift and weaken the effect, with manual downshifting being more noticeable. Fuel-injected vehicles have an additional advantage: the ECU cuts off fuel injection in this state, saving 0.4 liters of fuel per 100 kilometers.
Always remember that engine braking cannot replace the brakes! Once I was driving a manual transmission car down a steep slope in 2nd gear, and the rear wheels suddenly locked up when taking a turn. Later, the technician explained that because the gear was too high and the RPM didn't keep up, the engine resistance caused the drive wheels to lock up. Now I always lightly tap the brakes to reduce speed before downshifting. Be especially cautious on slippery roads, as front-wheel-drive cars are more prone to understeer. By the way, diesel engines, due to their high compression ratio, have a more pronounced effect than gasoline cars.
