Engine Braking

The Basic Principle

When the engine is spinning with the throttle closed, it has a tendency to slow down, mostly because of the compression effects in the cylinders. A coefficient is used in combination with an offset to generate a negative torque. This effect contributes significantly to braking in F1 cars and most other vehicles. The effect can be felt simply by driving and releasing the throttle, observing how quickly the engine tries to slow the car (the rolling resistance of the tires is generally a secondary effect).

Engine Braking in Formula 1

Understanding what happens within the engine when a driver lifts off the throttle is essential, as it has a subsequent effect on other aspects of the car. Unlike in road cars, an F1 driver does not leisurely lift off the throttle and delay the braking phase. Instead, the driver may be at near maximum revs when he simultaneously lifts off the throttle pedal completely and hits the brake pedal hard for the initial downforce-aided braking. During braking, the lower gears are sequentially selected, peaking revs all the time as the car slows down.

This sudden closing of the throttles blocks the inlet to the combustion chamber, but the pistons in the cylinders continue to pump up and down at great speed. This creates huge stresses inside the combustion chamber, and the resulting vacuum sucks air past the piston rings (so-called blow-by). This rapidly slows the engine, creating an excessive engine braking effect, which in turn stresses the drive train and over-brakes the engine. The excessive engine braking effect makes the car nervous on throttle lift-off, regardless of any subsequent aerodynamic effect. Engine manufacturers therefore find different solutions to ease the stresses and braking effect by modifying settings in the engine torque map.

In the past, there were several different engine strategies in place, and the driver was able to change the off-throttle overrun setting to tune the car’s handling. Drivers switching between teams often found that the change in overrun settings required adjustments to both their driving style and sometimes to the engine settings.

Manufacturer Solutions

Renault

The Renault engine, for example, ran the throttle open at 50% on the overrun (but with no fuel injected or spark). This both eased the blow-by, reducing the vacuum effect inside the pistons and the associated stress issues, and was also useful for cooling the exhaust valves. This approach is commonly known as cold-blown mapping.

Mercedes

Mercedes High Performance Engines employed a different solution known as fired or hot overrun. When the driver lifted off, fuel continued to be injected into the engine and the spark was fired within the combustion chamber, but ignition was delayed by as much as 45%. This offset the engine braking effect, giving a smoother transition from on-throttle to the overrun, while also reducing the vacuum effect inside the pistons and stress issues. As a result, there was less engine braking effect. This gave Mercedes the freedom to define braking bias and KERS charging without having to account for engine braking.

Electronic Engine Braking Control

Engine braking in F1 (a topic widely discussed after the introduction of the common ECU in Formula One and the abolition of sophisticated engine braking systems during 2008) is really shorthand for a range of functions that the active differentials and engine management systems perform under braking.

First, under hard braking the differential is locked to stabilise the car in yaw. Honda attempted to do something similar with the front differential a few years earlier, but it was banned.

Second, the engine management was thought to allow tuning of the brake balance corner-by-corner, in that the off-throttle fuelling could be altered to provide full engine drag or none to the rear wheels only, changing the brake balance considerably. The cars could determine their position on track via sophisticated GPS systems.

Finally, the system could detect the onset and offset of rear wheel lock-up under braking and supply power to the rear wheels, controlling differential slip or input to individual tires, although no team ever officially confirmed this. If the control system detected any rear wheels locking during braking, the engine management would blip the throttle to prevent it.