Brind
02-09-2003, 11:03 PM
Written by Mook, also a member on here.
Engines gain extra power by having greater air/fuel mix in the cylinders. More air fuel = more bang = more power. A normally aspirated engine INSERT INTO post VALUES (n/a) sucks air in but the amount is limited as air pressure is only so great, thus with n/a engines, more capacity = more power. A turbo forces more air/fuel into the engine than is normally possible, hense the name forced induction, and can give small angines comparable power outsput to lager displacement engines.
A turbo unit consists of two parts. One half sit in the exhaust system and uses the escaping gases to create it's power by spinning a turbine INSERT INTO post VALUES (hense the name) . This is then transfered to the other half, in the compressor side, which then uses this power to spin up the compressor wheel which forces the air into the engine on the intake side. It's very efficient as it uses the engines own power to operate and can increase the output of the engine by upto and over twice as much in some cases. As an example, the Rover T16 2 litre engine creates 140bhp normally aspirated, but with the turbo added, running 8psi of boost, that raises to 200bhp. 1psi of boost is approx 8bhp. This would be roughly the same for most 2 litre cars running a small turbo on a basic level.
That's the easy bit. Now, without any control of this you would simply have more boost the higher rpm the engine runs as more exhaust gas spins it faster. This is where the wastegate comes in. As well as the turbo itself, there is a second 'bypass' part of the exhaust which misses out the turbo altogether, this allows the extra exhaust gases to be 'wasted' and is controlled in much the same way your throat controls whether stuff going in goes into your stomach or lungs. The wastegate is essentially a flap and is, in turn, controlled by a spring of a certain strength in a unit called the wastegate actuator. The spring will open above a certain pressure, and it this that allows the turbo to hold a steady level of boost throughout the rev range. It takes it's feed off a vacuum pipe in the inlet manifold which tells the wastegate actuator what the pressure in the system is. It's all done mechanically, and has a slight delay in it's working which causes overboost, or boost spikes - this is where the boost pressure is temporarily higher than it should be and isn't a good thing because it can cause engine problems.
So, we now know what a turbo is and how it's boost level is kept stable. Now, onto dump valves and their purpose in proceedings. When you change gear, or ease off the power, you don't really want to have too much pressure in the system, this can be vented with the dump valve. It works in a similar way to the wastegate in that it is spring loaded and works off the pressure in the manifold. When you change gear you lift off the acellerator which eases off the turbo and the manifold goes into vacuum again, this vacuum releases the dump valve, opening it up to vent off the excess pressure, creating the *tshhh* sound we all know and love. When you put your foot down again, pressure goes positive thus closing the DV and keeping the boost in system. Now, the general opinion is that all turbos need a dump valve to vent off excess pressure and stop the turbo from stalling with the build up of pressure in the system when you change gear. However, that's not the case. It is only engines with big turbos running high pressure INSERT INTO post VALUES (20psi or so and above) that need a DV to prevent them stalling. Smaller turbos on lower boost will not generate enough pressure to stall the turbo, so adding a DV to vent off the pressure will actually INCREASE turbo lag as it then has to spin up again from scratch. Let's not forget that the engine is still running sucking in air so the back pressure is minimal. This is why cars such as the RSTs came without one as standard, and we all know diesel turbos don't have them. The reason most manufacturers put recirculating DVs in the standard car is to stop it from making any noise and to try and increase the life of the turbo by removing any backpressure from the system. So, not having a DV could reduce turbo life, but no more so than increasing the boost a few psi, so it's not a serious problem running without one. It's also the case that without a DV, you will hear turbo flutter upon changing gear as the compressor wheel chops the air coming back, this is the sound of the great Group B rally cars and is way cooler than any DV
On to upping the boost. To do this you need to keep the wastegate open more/for longer. It can be done with a stronger spring in the wastegate actuator itself, or more commonly by a device that sits in the system between the manifold and wastegate actuator. The device will do is bleed off pressure. The most basic form of such a device is a bleed valve, which you add in and open slightly to vent off some of the pressure from the inlet manifold, for example 3psi. This means that when the wastegate actuator thinks it's 8psi and closes, it's actually 11psi, so you've got 3psi more of boost. At the top end of the scale you can get electronic boost controllers running with instant response solenoid valves. These can be set to various levels and have all sorts of functions. The Apexi AVCR is one of the best of these devices, allowing you to run differing levels of boost over the rev range and between gears! The electronic devices also remove the boost spikes mentioned above, keeping things safer.
Now intercoolers. We know more air/fuel into engine = more power. We also know that cold air is denser than warm air and for the same space has more oxygen in it. So, the colder the air, the more power your turbo adds. Hense the adding of the intercooler between the turbo and inlet manifold. The turbo is spinning away frantically creating friction and pressure, which create heat. Without an intercooler the temp of the air entering the engine could get upto and over 100 deg C, which is unhealthy, as it causes the pre-detonation INSERT INTO post VALUES (or pinking). So, the intercooler, much like a radiator, cools the air going from the turbo to the engine, making it even more efficient. You'll hear people talk of bigger and better intercoolers, all with the aim of cooling the air to keep it denser to get more air/fuel into the engine.
At the end of the day, you can only increase power by adding more air/fuel into the cylinders to burn. It's as simple as that
Engines gain extra power by having greater air/fuel mix in the cylinders. More air fuel = more bang = more power. A normally aspirated engine INSERT INTO post VALUES (n/a) sucks air in but the amount is limited as air pressure is only so great, thus with n/a engines, more capacity = more power. A turbo forces more air/fuel into the engine than is normally possible, hense the name forced induction, and can give small angines comparable power outsput to lager displacement engines.
A turbo unit consists of two parts. One half sit in the exhaust system and uses the escaping gases to create it's power by spinning a turbine INSERT INTO post VALUES (hense the name) . This is then transfered to the other half, in the compressor side, which then uses this power to spin up the compressor wheel which forces the air into the engine on the intake side. It's very efficient as it uses the engines own power to operate and can increase the output of the engine by upto and over twice as much in some cases. As an example, the Rover T16 2 litre engine creates 140bhp normally aspirated, but with the turbo added, running 8psi of boost, that raises to 200bhp. 1psi of boost is approx 8bhp. This would be roughly the same for most 2 litre cars running a small turbo on a basic level.
That's the easy bit. Now, without any control of this you would simply have more boost the higher rpm the engine runs as more exhaust gas spins it faster. This is where the wastegate comes in. As well as the turbo itself, there is a second 'bypass' part of the exhaust which misses out the turbo altogether, this allows the extra exhaust gases to be 'wasted' and is controlled in much the same way your throat controls whether stuff going in goes into your stomach or lungs. The wastegate is essentially a flap and is, in turn, controlled by a spring of a certain strength in a unit called the wastegate actuator. The spring will open above a certain pressure, and it this that allows the turbo to hold a steady level of boost throughout the rev range. It takes it's feed off a vacuum pipe in the inlet manifold which tells the wastegate actuator what the pressure in the system is. It's all done mechanically, and has a slight delay in it's working which causes overboost, or boost spikes - this is where the boost pressure is temporarily higher than it should be and isn't a good thing because it can cause engine problems.
So, we now know what a turbo is and how it's boost level is kept stable. Now, onto dump valves and their purpose in proceedings. When you change gear, or ease off the power, you don't really want to have too much pressure in the system, this can be vented with the dump valve. It works in a similar way to the wastegate in that it is spring loaded and works off the pressure in the manifold. When you change gear you lift off the acellerator which eases off the turbo and the manifold goes into vacuum again, this vacuum releases the dump valve, opening it up to vent off the excess pressure, creating the *tshhh* sound we all know and love. When you put your foot down again, pressure goes positive thus closing the DV and keeping the boost in system. Now, the general opinion is that all turbos need a dump valve to vent off excess pressure and stop the turbo from stalling with the build up of pressure in the system when you change gear. However, that's not the case. It is only engines with big turbos running high pressure INSERT INTO post VALUES (20psi or so and above) that need a DV to prevent them stalling. Smaller turbos on lower boost will not generate enough pressure to stall the turbo, so adding a DV to vent off the pressure will actually INCREASE turbo lag as it then has to spin up again from scratch. Let's not forget that the engine is still running sucking in air so the back pressure is minimal. This is why cars such as the RSTs came without one as standard, and we all know diesel turbos don't have them. The reason most manufacturers put recirculating DVs in the standard car is to stop it from making any noise and to try and increase the life of the turbo by removing any backpressure from the system. So, not having a DV could reduce turbo life, but no more so than increasing the boost a few psi, so it's not a serious problem running without one. It's also the case that without a DV, you will hear turbo flutter upon changing gear as the compressor wheel chops the air coming back, this is the sound of the great Group B rally cars and is way cooler than any DV
On to upping the boost. To do this you need to keep the wastegate open more/for longer. It can be done with a stronger spring in the wastegate actuator itself, or more commonly by a device that sits in the system between the manifold and wastegate actuator. The device will do is bleed off pressure. The most basic form of such a device is a bleed valve, which you add in and open slightly to vent off some of the pressure from the inlet manifold, for example 3psi. This means that when the wastegate actuator thinks it's 8psi and closes, it's actually 11psi, so you've got 3psi more of boost. At the top end of the scale you can get electronic boost controllers running with instant response solenoid valves. These can be set to various levels and have all sorts of functions. The Apexi AVCR is one of the best of these devices, allowing you to run differing levels of boost over the rev range and between gears! The electronic devices also remove the boost spikes mentioned above, keeping things safer.
Now intercoolers. We know more air/fuel into engine = more power. We also know that cold air is denser than warm air and for the same space has more oxygen in it. So, the colder the air, the more power your turbo adds. Hense the adding of the intercooler between the turbo and inlet manifold. The turbo is spinning away frantically creating friction and pressure, which create heat. Without an intercooler the temp of the air entering the engine could get upto and over 100 deg C, which is unhealthy, as it causes the pre-detonation INSERT INTO post VALUES (or pinking). So, the intercooler, much like a radiator, cools the air going from the turbo to the engine, making it even more efficient. You'll hear people talk of bigger and better intercoolers, all with the aim of cooling the air to keep it denser to get more air/fuel into the engine.
At the end of the day, you can only increase power by adding more air/fuel into the cylinders to burn. It's as simple as that