If you own or drive a manual transmission equiped vehicle on a regular basis you use engine braking more than you realize. But alot of people don't know how or why engine braking occures.
I'm writing this article to explain how engine braking works and to explain away alot of the misconceptions surrounding this phenomenon.
Common misconceptions consist of the following:
-Air in the crank case causing resistance
-Friction in the engine causing resistance
-Compression
I will start by stating how an engine actualy intakes air, because that is an important part of how engine braking actualy works.
An engine idles because the intake is restricted so that the vacuum is just enough to keep the engine at a fixed speed but not enough to stop the engine completely. An engine is in a sense an air scavenger, it will take in as much air as the intake system will allow.
If you completely removed the throttle butterfly, or even just opened it completely with the engine unloaded, it would accelerate untill it could not take in any more air provided there is sufficient fuel. Unfortunately this is always far faster than the engine was designed for and it will probably rev itself apart in a short time.
This is actualy how restrictor plates work as well, they restrict the extra overhead that the intake system would normaly allow. This means that as the engine hits higher speeds it actualy starts to run out of air and this causes a reduction in power. Normaly it is not enough to cut into the redline of the engine, but in extreme use it can.
Now on to the misconceptions.
-Air in the crank case causing resistance
Air in the crank moves freely around above the oil in the pan and is unrestricted, there is usualy a vacuum line hooked up to the crank case vent to prevent air and gasses from building up.
The air mass causes very little resistance because as one piston moves down there is always another moving up. So the air in the crank never gets compressed, only pushed around. An engine can always overcome the effort needed to move this air mass, even with the throttle closed.
-Friction in the engine causing resistance
Friction does have a very small part in lowering engine speed with the throttle off, but it is no where near enough to cause the heavy braking effect seen in the lower gears on some vehicles. If it where that intense the engine would simply stop rotating completely.
-Compression
This is probably the most common misconception, and people also use the wrong term in conjunction with it. Compression acts as a spring, if it isn't released it will return most of the energy it takes to complete. Not to mention that in multiple cylinder engines it is almost always paired with a power stroke.
Compression braking is only seen in diesel engines, and it works only because of special equipment added to that engine. More on this after I explain how engine braking on gasoline burning engines works.
Now onto how it actualy works.
Engine braking happens when an engine is reved up to a higher than idle speed and then the throttle is closed, since the engine is essentialy an air scavenger it will still try to take in enough air to maintain it's current speed. Because the throttle is closed it will generate a vacuum in the intake system and even in the cylinders.
The engine is now generating an opposite force, instead of applying torque in the forward direction it is resisting the rotation of the transmission, it is trying to slow down because the vacuum in the intake and cylinders is acting on the pistons with much greater force.
The vacuum will increase or decrease as engine speed climbs or falls, if you rev it higher and release the throttle it will generate more braking force than at lower RPMs, additionaly, because of the force multiplying of the gears in the transmission you will experiance more engine braking in lower gears.
This is also largely effected by the shape of the cylinders, for instance:
You have two engines, both are 1 litre with four cylinders. The first one has a short stroke, but a large bore. The second has a long stroke and a narrower bore. Both still have the same displacement but are of different shapes. Intake and exhaust are identical.
Provided the transmission ratios are the same, the short stroke engine will always experiance more engine braking in any given gear than the long stroke engine. This is because the surface area the vacuum can act on is larger in the short stroke engine because the pistons have a larger surface area.
How diesels achive engine braking is a bit more complicated.
A diesel engine has no butterfly valve in the throttle body, they accelerate or slow down by increasing or decreasing the amount of fuel injected into the cylinders. There is no vacuum to slow the engine down when the throttle is released, it slows simply because combustion is insufficient to maintain speed without the fuel from an open throttle.
Compression in a diesel works exactly the same as in a gasoline engine, it returns most of the energy it takes when it springs back. However, because there is no vacuum there is nothing preventing the engine from accelerating on a down hill section of road even when the throttle is closed.
This was and sometimes still is a problem for trucks, so several special devices where invented to prevent the problem.
Most work on the same principal but go by different names, I will simply call them compression release brakes.
A compression release brake does exactly that, it releases the compression before it has a chance to return the energy back to the vehicle. You've probably heard it before, a machine gun like sound from a transport slowing down. Even with mufflers installed a compression brake is very loud. Loud enough that I have wittnessed drivers using them as horns to scare slow drivers infront of them.
Another device used less commonly on diesels is an exhaust restrictor, it essentialy increases the pressure in the exhaust system puting resistance on the exhaust stroke and thus reducing the speed of the vehicle.
I hope this clears up some of the mystery surrounding engine braking, and I may revise and add to this if I find I missed anything. Also, feel free to ask questions. I realy want to squash the myths surrounding this because I hear them on a regular basis.
I'm writing this article to explain how engine braking works and to explain away alot of the misconceptions surrounding this phenomenon.
Common misconceptions consist of the following:
-Air in the crank case causing resistance
-Friction in the engine causing resistance
-Compression
I will start by stating how an engine actualy intakes air, because that is an important part of how engine braking actualy works.
An engine idles because the intake is restricted so that the vacuum is just enough to keep the engine at a fixed speed but not enough to stop the engine completely. An engine is in a sense an air scavenger, it will take in as much air as the intake system will allow.
If you completely removed the throttle butterfly, or even just opened it completely with the engine unloaded, it would accelerate untill it could not take in any more air provided there is sufficient fuel. Unfortunately this is always far faster than the engine was designed for and it will probably rev itself apart in a short time.
This is actualy how restrictor plates work as well, they restrict the extra overhead that the intake system would normaly allow. This means that as the engine hits higher speeds it actualy starts to run out of air and this causes a reduction in power. Normaly it is not enough to cut into the redline of the engine, but in extreme use it can.
Now on to the misconceptions.
-Air in the crank case causing resistance
Air in the crank moves freely around above the oil in the pan and is unrestricted, there is usualy a vacuum line hooked up to the crank case vent to prevent air and gasses from building up.
The air mass causes very little resistance because as one piston moves down there is always another moving up. So the air in the crank never gets compressed, only pushed around. An engine can always overcome the effort needed to move this air mass, even with the throttle closed.
-Friction in the engine causing resistance
Friction does have a very small part in lowering engine speed with the throttle off, but it is no where near enough to cause the heavy braking effect seen in the lower gears on some vehicles. If it where that intense the engine would simply stop rotating completely.
-Compression
This is probably the most common misconception, and people also use the wrong term in conjunction with it. Compression acts as a spring, if it isn't released it will return most of the energy it takes to complete. Not to mention that in multiple cylinder engines it is almost always paired with a power stroke.
Compression braking is only seen in diesel engines, and it works only because of special equipment added to that engine. More on this after I explain how engine braking on gasoline burning engines works.
Now onto how it actualy works.
Engine braking happens when an engine is reved up to a higher than idle speed and then the throttle is closed, since the engine is essentialy an air scavenger it will still try to take in enough air to maintain it's current speed. Because the throttle is closed it will generate a vacuum in the intake system and even in the cylinders.
The engine is now generating an opposite force, instead of applying torque in the forward direction it is resisting the rotation of the transmission, it is trying to slow down because the vacuum in the intake and cylinders is acting on the pistons with much greater force.
The vacuum will increase or decrease as engine speed climbs or falls, if you rev it higher and release the throttle it will generate more braking force than at lower RPMs, additionaly, because of the force multiplying of the gears in the transmission you will experiance more engine braking in lower gears.
This is also largely effected by the shape of the cylinders, for instance:
You have two engines, both are 1 litre with four cylinders. The first one has a short stroke, but a large bore. The second has a long stroke and a narrower bore. Both still have the same displacement but are of different shapes. Intake and exhaust are identical.
Provided the transmission ratios are the same, the short stroke engine will always experiance more engine braking in any given gear than the long stroke engine. This is because the surface area the vacuum can act on is larger in the short stroke engine because the pistons have a larger surface area.
How diesels achive engine braking is a bit more complicated.
A diesel engine has no butterfly valve in the throttle body, they accelerate or slow down by increasing or decreasing the amount of fuel injected into the cylinders. There is no vacuum to slow the engine down when the throttle is released, it slows simply because combustion is insufficient to maintain speed without the fuel from an open throttle.
Compression in a diesel works exactly the same as in a gasoline engine, it returns most of the energy it takes when it springs back. However, because there is no vacuum there is nothing preventing the engine from accelerating on a down hill section of road even when the throttle is closed.
This was and sometimes still is a problem for trucks, so several special devices where invented to prevent the problem.
Most work on the same principal but go by different names, I will simply call them compression release brakes.
A compression release brake does exactly that, it releases the compression before it has a chance to return the energy back to the vehicle. You've probably heard it before, a machine gun like sound from a transport slowing down. Even with mufflers installed a compression brake is very loud. Loud enough that I have wittnessed drivers using them as horns to scare slow drivers infront of them.
Another device used less commonly on diesels is an exhaust restrictor, it essentialy increases the pressure in the exhaust system puting resistance on the exhaust stroke and thus reducing the speed of the vehicle.
I hope this clears up some of the mystery surrounding engine braking, and I may revise and add to this if I find I missed anything. Also, feel free to ask questions. I realy want to squash the myths surrounding this because I hear them on a regular basis.
