What are plunum, throttle bodies and a bit more?
Discussion
Please help - the question of plenum over throttle bodies comes up all the time here. This leads me to think I'm really missing something...
As I understand it:
A throttle body contains a throttle mechanism, its primary purpose is to control the flow of mix from its inlet to its outlet by limiting it from a maximum.
In a petrol engine this is required so the engine doesn't run at full intake rate all the time meaning you'd end up either not being able to start up or constantly accelerate at maximum rate.
You can have 1 throttle for all cylinders in the engine upto any number of throttles per cylinder (I'd guess a realistic limit would be 1 per cylinder, but I have no reason to believe this is a physical requirement).
A plenum is a positive pressure of a fluid (the antithesis of vacuum).
On a car a plenum chamber is used to duct positive pressure intake charge evenly across the cylinder ports (compared to valve pressure I guess). This requires more than a length of pipe from the air filter through the throttle to the port, so is designed for the specific installation (otherwise a single ducting system would result in uneven pressure across cylinder intake ports, even on the same cylinder).
If you remove the plenum chamber, you still need to get mix through the throttles into the cylinders.
If we have an induction system per cylinder port and we can run each system to allow the specific throttle path to the port to run as required, we're good, but need to tune and match all of them.
On a normally asperated 4 stroke with no overlap in the valve timing, the intake stroke causes a lower pressure in the cylinder and the atmospheric pressure outside of it pushes mix through the value into the cylinder.
With some overlap, the theory is that the exhaust pulse has already exited the cylinder and making its merry way down the exhaust system to the big wide world. While it is in the exhaust pipe, it pulls along/leaves behind a lower pressue volume that aids the push of mix into the cylinder.
I understand that the exhaust leaves the cylinder very quickly and that by the time the intake valve opens, there is little to no exhaust to 'wash back' up the intake line (and if there was, it would be against the idea of the overlap).
The only problem I hear of with these cams is rough running at low RPM...
I have no reason to believe the exhaust gas knows what speed the engine is running, so will take the same amount of time to get down the exhaust pipe at high RPM as it would at low RPM. Unless the following exhaust pulse pushes the previous, cooling, exhuast pulse out quicker - and at high RPM there are more exhaust pulses per time interval too, so this could be the case (I have no idea).
The actual time (not crank angle) the valve opening overlap happens for will be increased with low rpm, but the valve configuration would be the same as at high RPM (unless you have a variable cam mechinism), so the only thing that would happen differently is that the mix has more time to enter the cylinder, and then, possibly, be dragged out again down the exhaust pipe to follow any exhaust pulse that is still in the exhuast pipe (though I would hope a new car had the exhaust length tuned to minimise this). As we're running at low RPM, we have a closed throttle, so less mix anyway, this would leave even less mix in the cylinder to burn, so less BMEP on the piston head, so less torque, etc, etc... but it would be like running the engine at a lower RPM than it is really running at - can't you just set the idle throttle opening larger to compensate for this?
I do not see how removing the plenum chamber would help with this? You still need an even distribution of pressure at the throttle inlet (if not the cylinder ports themselves) and allow for the opening and closing of the valves leading to variable pressure requirements.
Please spark the light for me.
Google: What is a throttle body?
Google: What is a plenum?
As I understand it:
A throttle body contains a throttle mechanism, its primary purpose is to control the flow of mix from its inlet to its outlet by limiting it from a maximum.
In a petrol engine this is required so the engine doesn't run at full intake rate all the time meaning you'd end up either not being able to start up or constantly accelerate at maximum rate.
You can have 1 throttle for all cylinders in the engine upto any number of throttles per cylinder (I'd guess a realistic limit would be 1 per cylinder, but I have no reason to believe this is a physical requirement).
A plenum is a positive pressure of a fluid (the antithesis of vacuum).
On a car a plenum chamber is used to duct positive pressure intake charge evenly across the cylinder ports (compared to valve pressure I guess). This requires more than a length of pipe from the air filter through the throttle to the port, so is designed for the specific installation (otherwise a single ducting system would result in uneven pressure across cylinder intake ports, even on the same cylinder).
If you remove the plenum chamber, you still need to get mix through the throttles into the cylinders.
If we have an induction system per cylinder port and we can run each system to allow the specific throttle path to the port to run as required, we're good, but need to tune and match all of them.
On a normally asperated 4 stroke with no overlap in the valve timing, the intake stroke causes a lower pressure in the cylinder and the atmospheric pressure outside of it pushes mix through the value into the cylinder.
With some overlap, the theory is that the exhaust pulse has already exited the cylinder and making its merry way down the exhaust system to the big wide world. While it is in the exhaust pipe, it pulls along/leaves behind a lower pressue volume that aids the push of mix into the cylinder.
I understand that the exhaust leaves the cylinder very quickly and that by the time the intake valve opens, there is little to no exhaust to 'wash back' up the intake line (and if there was, it would be against the idea of the overlap).
The only problem I hear of with these cams is rough running at low RPM...
I have no reason to believe the exhaust gas knows what speed the engine is running, so will take the same amount of time to get down the exhaust pipe at high RPM as it would at low RPM. Unless the following exhaust pulse pushes the previous, cooling, exhuast pulse out quicker - and at high RPM there are more exhaust pulses per time interval too, so this could be the case (I have no idea).
The actual time (not crank angle) the valve opening overlap happens for will be increased with low rpm, but the valve configuration would be the same as at high RPM (unless you have a variable cam mechinism), so the only thing that would happen differently is that the mix has more time to enter the cylinder, and then, possibly, be dragged out again down the exhaust pipe to follow any exhaust pulse that is still in the exhuast pipe (though I would hope a new car had the exhaust length tuned to minimise this). As we're running at low RPM, we have a closed throttle, so less mix anyway, this would leave even less mix in the cylinder to burn, so less BMEP on the piston head, so less torque, etc, etc... but it would be like running the engine at a lower RPM than it is really running at - can't you just set the idle throttle opening larger to compensate for this?
I do not see how removing the plenum chamber would help with this? You still need an even distribution of pressure at the throttle inlet (if not the cylinder ports themselves) and allow for the opening and closing of the valves leading to variable pressure requirements.
Please spark the light for me.
Google: What is a throttle body?
Google: What is a plenum?
Sort of - I guess if removing the plenum requires the fitting of throttle bodies then I am missing something.
AFAIK the plenum doesn't throttle the mix it controls its pressure, and throttle bodies are just housings for throttles which control the flow but not the pressure (with a couple of extra bits). Together they do both jobs - control the mix charge pressure and flow, meaning the driver has an "accelerator" to control the speed of the car with.
How can an engine run properly without these two control systems in the first place? Surely there is some flow and pressure control on all conventional engines? I would assume - from the posts I see here - that most cars have plenum chambers, but not throttles (or at least not ones that have bodies, so no idea how they manage to control flow?!).
I really must be missing something... I'll dig around the internet at lunch for the intake side of 4-stroke engines, but I guess there are so many configurations that I won't find a direct answer (especially as I think I don't know the question as I am missing something)
AFAIK the plenum doesn't throttle the mix it controls its pressure, and throttle bodies are just housings for throttles which control the flow but not the pressure (with a couple of extra bits). Together they do both jobs - control the mix charge pressure and flow, meaning the driver has an "accelerator" to control the speed of the car with.
How can an engine run properly without these two control systems in the first place? Surely there is some flow and pressure control on all conventional engines? I would assume - from the posts I see here - that most cars have plenum chambers, but not throttles (or at least not ones that have bodies, so no idea how they manage to control flow?!).
I really must be missing something... I'll dig around the internet at lunch for the intake side of 4-stroke engines, but I guess there are so many configurations that I won't find a direct answer (especially as I think I don't know the question as I am missing something)
Foolish Dave said:
How can an engine run properly without these two control systems in the first place? Surely there is some flow and pressure control on all conventional engines? I would assume - from the posts I see here - that most cars have plenum chambers, but not throttles (or at least not ones that have bodies, so no idea how they manage to control flow?!).
I think you're over-thinking this. The throttle is just a variable restricter that can reduce the air flow into the engine (reduce the engine volumetric efficiency). All petrol engined cars will have a throttle, usually just a single one at the entry to a common plenum, sometimes one per bank, occasionally one per cylinder. Diesels don't need throttles although sometimes they do have them too.Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff