Today The resulting vehicles are what we refer to as compliance cars.
Gasoline engines[ edit ] The term "engine braking" refers to the braking effect that occurs in gasoline engines when the accelerator pedal is released. This results in the throttle valve that controls intake airflow closing and the air flow through the intake becoming greatly restricted but not cut off completely.
This causes a high manifold vacuum which the cylinders have to work against—sapping energy and producing the majority of the engine braking force.
While some of the braking force is produced due to friction in the drive trainthis is negligible compared to the effect from the manifold vacuum caused by the air-flow restriction.
Diesel engines[ edit ] Diesel engines do not have engine braking in the above sense. Unlike gasoline engines, diesel engines vary fuel flow to control power, rather than throttling air intake and maintaining a constant fuel ratio as gasoline engines do.
Since they do not maintain a throttle vacuum, they are not subjected to the same engine braking effects. This is partly why non-turbo diesel-engined vehicles can coast in-gear for longer than an equivalent gasoline engine. The higher compression ratio in diesels means they are harder to start, but once they are running the energy expended in compressing air is regained during the expansion stroke when the compressed air is allowed to "spring" back, so the higher compression ratio causes negligible engine braking via energy being Why diesels make sense as friction and heat of compressed air to engine block.
Compression release brake[ edit ] A compression release brake also known as a Jacobs brake or "jake brake" Why diesels make sense, is the type of brake most commonly confused with real engine braking; it is used mainly in large diesel trucks and works by opening the exhaust valves at the top of the compression stroke, so the large amount of energy stored in that compressed air is not returned to the crankshaft, but is released into the atmosphere.
Normally, during the compression stroke, energy is used as the upward-traveling piston compresses air in the cylinder; the compressed air then acts as a compressed spring and pushes the piston back down.
However, with the jake brake in operation, the compressed air is suddenly released just before the piston begins its downward travel. This sudden release of compressed air creates audible sound waves similar to the expanding gases escaping from the muzzle of a firearm.
Having lost the energy stored within the compressed air, there is no "spring back" so the engine must expend yet more energy pulling the piston back down again. This type of brake is banned or restricted in many locations where people live because it creates a machine gun like sound loud enough to disturb the peace, including waking people at night.
In simple terms, it works by increasing the back-pressure of the exhaust. Nearly all of these brakes are butterfly valves similar to a throttle valve, mounted downstream of the turbocharger if there is one. Exhaust obstructions[ edit ] Modern diesels are subject to many strict controls on emissions and often have many obstructions in the exhaust, which cause them to feel like they have some engine braking like a gasoline engine.
The main ones are: Diesel particulate filter DPF is designed to capture soot particles that would otherwise be released into the atmosphere; it greatly obstructs exhaust flow and can sap almost as much power as a small air conditioning pump. Two-stroke engines[ edit ] Engine braking in a premix two-stroke engine can be extremely harmful to the engine, because cylinder and piston lubricant is delivered to each cylinder mixed with fuel.
Consequently, during engine braking, the engine starves not only of fuel but also lubricant, causing accelerated wear. Many old two-stroke cars Saab, Wartburg, etc. Most two-stroke motorcycle engines since the s have had lubrication by an oil pumpindependent of the throttle and fuel system, such as Suzuki's Posi-Force system.
Applications[ edit ] As soon as the accelerator is released enough to slow the engine, engine braking comes into effect as long as the wheels remain connected via the transmission to the engine. A slipping or disengaged clutchor a torque converterwould disengage the wheels or absorb braking energy.
The braking force varies depending on the engine, and the gear the transmission is in.
The lower the gear, the higher the braking effect due to higher rpm and the torque transferred through the transmission higher torque is delivered from the engine in lower gears. Engine braking avoids wear on brakes, and can help a driver maintain control of the vehicle.
Active use of engine braking by shifting into a lower gear can help control speed while driving down very steep and long slopes, saving the brakes from overheating or excessive wear.
If it is applied before the brakes have been used, it can leave the brakes available to make emergency stops. The desired speed is maintained by using engine braking to counteract gravitational acceleration.Find the best diesel fuel additive for your vehicle!
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