back pressure (normally aspirated)

can anybody explain why some say a normally aspirated engine needs a certain amount of back pressure (positive pressure in the manifold outside the exhaust port/valve) is important on a normally aspirated motor.

I can see that it might work in a way a bit like the TT's EGR ( ? exhaust gas recirc) in that some exhaust gas could be left in the cylinder... but umm.. why?

other suggestion I have read is that without the back pressure, the gases exit so fast causing the valves to get far too hot. But then why isn't this a problem on force fed motors?

Other suggestion I have read is that on a car with poorly designed cam timings where the inlet and exhaust valves overlap (are momentarily open at the same time to a degree) then the lack of back pressure (or in fact negative pressure due to car moving fast or even syphoning-type phsyics) can cause the incoming fuel/air mix to be buggered (like scavenged).

or something.

hmmm. Ian C ? You there?

Originally posted by carl0s

can anybody explain why some say a normally aspirated engine needs a certain amount of back pressure (positive pressure in the manifold outside the exhaust port/valve) is important on a normally aspirated motor.

I'll have a go...

I can see that it might work in a way a bit like the TT's EGR ( ? exhaust gas recirc) in that some exhaust gas could be left in the cylinder... but umm.. why?

Not really like EGR (see my EGR FAQ entry in the sticky thread at the top of technical). If you want to get cutting-edge, then you can trap heat in the combustion chamber to initiate the next burn in a CAI (Controlled Auto Ignition) gasoline engine, but that's another story entirely...

other suggestion I have read is that without the back pressure, the gases exit so fast causing the valves to get far too hot. But then why isn't this a problem on force fed motors?

Hmmmm. Not too sure in this one. Soz.

Other suggestion I have read is that on a car with poorly designed cam timings where the inlet and exhaust valves overlap (are momentarily open at the same time to a degree) then the lack of back pressure (or in fact negative pressure due to car moving fast or even syphoning-type phsyics) can cause the incoming fuel/air mix to be buggered (like scavenged).

I think that this is probably 80% of your answer right here. Careful with the use of "poorly designed" though. Valve overlap is very common, especially in race engines which is why they pop and fart so much and don't idle worth a damn. Just like you can tune an intake system to that the standing waves in the runners can "ram" air into the plenum, you can tune the exhaust runners and back pressure so that it helps suck out the exhaust gases, or helps hold the intake charge in the cylinder even when the valve is open.

or something.

 

hmmm. Ian C ? You there?

Or something indeed. This isn't really my area of expertese. I probably have a book somewhere that could explain it fully...

Originally posted by Darren Blake Just like you can tune an intake system to that the standing waves in the runners can "ram" air into the plenum, you can tune the exhaust runners and back pressure so that it helps suck out the exhaust gases, or helps hold the intake charge in the cylinder even when the valve is open.

Isn't that called pulse tuning and can only be set to be optimum at one engine speed. On the intake side sime cars have two sets of different length pipes that are tuned to a specific speed.

Mini's used to have an LCB (long centre branch) exhaust that helped time the exhaust pulses to keep a steady stream.

I've never quite understood back pressure though, unless it's to do with sucking the gasses out, and hence pulling in the intake charge during the overlap.

a well designed exhaust can improve volumetric efficiency(VE) on a N/A engine as well as supercharged ones.

used to work for an engineering company designing exhaust piping for marine engines (diesel) but the principle is similar for cars.

The equations can be found

here

Not really required for understanding but basically anything to improve VE will aid the creation of horsepower.

With a well designed exhaust you get..

Escaping exhaust gas creates pressure wave moving down the system.

Wave hits the system’s end, causing “suction wave” to travel back up the system.

Suction wave hits the cylinder and reverses direction just as the exhaust valve opens.

The suction wave helps pull exhaust gases from the cylinder improving engine breathing and volumetric efficiency.

Very difficult to design for all RPM's - exhausts can be designed for high end power or lower end torque. Optimum VE rpm occurs at peak torque rpm.

All the above goes out of the window somewhat with turbocharging as peak VE can be just about anywhere you choose by controlling the boost map. Most modern N/A cars operate at about 80-90% VE with 100% being very good and 110% achievable without super/turbocharging with very specialised design on the intake, valves, pistons, exhaust etc.

F1 engine designers are obsessed with VE as the only way to get more HP (ignoring mechanical losses) is to get more air into the cylinders and hence more fuel as they have to keep the A/F stoich.

Too much info perhaps - this subject occupied my working life for 2 years....

ATB

ok I think I'm partly understanding what you are saying.

I can see that holding the intake charge in the chamber is the function that back pressure provides.

Sucking exhaust gases out is the exact opposite though, and is my reason for asking. If you want the exhaust gases sucked out, you mustn't need to hold the intake charge in the chamber ('cause the vales don't overlap), then you will want NO back pressure, in fact you'll want negative pressure there.

Sounding half-right?

All exhaust pipes produce SOME back pressure, but you are confusing a none existent need for back pressure in an N/A engine with a desire for pulse wave tuned length of inlet and exhaust. Back pressure per se in turbo or N/A exhausts is undesirable. Of course a turbo itself produces a huge amount of back pressure when exhaust volumes are high, and is the reason hybrid turbos are a bit of a cock up when the exhaust housing size is already very small and just a bigger compressor side is added.

Originally posted by Chris Wilson

All exhaust pipes produce SOME back pressure, but you are confusing a none existent need for back pressure in an N/A engine

So we're decided then, back pressure is *not* needed in an N/A engine?

The reason I asked was because I had always wondered what the purpose of back pressure was, and now people are saying in another thread that a 3" de-cat pipe is too large for an NA engine because all back pressure will be lost.

Ok, two reasons for asking so far then

I think with a decat and a free flowing exhaust on an NA the performance will be effected at points across the rev range range.

At some point the performance will be less and at another point (where in the rev range i dont know, probably higher up) the perfomance will improve slightly.

This is due to the back pressure, or lack of it, as the case may be. I always thought that the requirement for back pressure on an NA was to help suck the gases through the system.

If you dont have any back pressure can you create a vacum in the cylinder?

but that's the point I'm trying to understand. Back pressure would surely impede the extraction of exhaust gases.

No back pressure, or a vacuum, would 'extract'.

No beacuse the original exhasut is carefully designed for a reason. It also probably has something to do with these pulse waves:conf:

I don't know if its possible but a vaccum in the cylinder may cause more fuel or air to be drawn out than required.

I wouldn't think its possible with the fuel as this is controlled by the injectors.

It's all to do with pulse wave tuning, as has been said already. As the exhaust exits the cylinder it helps suck in the intake charge - but only in a narrow rpm range. You need valve overlap for this, the exhaust valve has to still be open while the intake valve opens. The side effect of this is that when you are out of the tuned rpm range the exhaust gas comes back in and a) corrupts the fresh inlet charge and b) slows down the incoming inlet charge. So, for road vehicles you have very mild cams. For race engines you have wild cams that can't idle but have extreme pulse tuning to boost their VE at full chat.

Putting a huuuge zorst on an NA can remove all this careful tuning and can make the engine weaker at the point it usually felt strongest but doesn't stop the valve overlap's negative effects... Most of the pulse tuning is, and correct me if I'm wrong, done in the first 24" of the exhaust system though, so a big back box makes little difference.

Turbo engines don't need huge valve overlap or pulse tuning as they ram the air in anyway. Hence the term forced induction. In fact, a wild cam overlap means part of the intake charge scuttles straight through the cylinder into the exhaust pipe! Not only is this wasted intake air, but you run rich because you lose metered air, and then you run lean because your wideband O2 sensor picks up the extra oxygen in the exhaust

-Ian

I did you a favour by posting up Carlos, I knew Ian would be along to contradict me. He follows me:p

In fact, a wild cam overlap means part of the intake charge scuttles straight through the cylinder into the exhaust pipe! Not only is this wasted intake air, but you run rich because you lose metered air, and then you run lean because your wideband O2 sensor picks up the extra oxygen in the exhaust

Ian, what does this actually mean then? Do the two cancel each other out? Doesn't the O2 sensor have an tolerance built in to avoid the ECU making rapid changes to the air intake, or does the ECU allow take an average reading and ratio the air accordingly?

Theoretically you'll run rich with a wild cam under high boost. Under boost you'll be in open loop mode, so a) the ECU uses it's map and ignores the stock narrowband O2 sensor input, and b) it wouldn't be using a wideband input anyway.

What I mean is, if you are trying to map the car, you'll see it leaner than it is as you are getting unburnt air coming through and mixing into the exhaust, yet you'll really be running too rich because part of the metered air charge is being lost in this fashion. Depending on when it's injected some fuel may go with it. But then the unburnt fuel would change the wideband O2 reading as well. Nightmare for tuning basically A wild cam isn't required for turbo engines (although I've no idea if it is useful in the upper echelons of motorsport like CART and the turbo F1 days etc)

Anyway, that's really offtopic now so I'll shut up

-Ian

thanks for all the input guys. I'm not so sure I understand entirely but it's interesting reading all the same

Originally posted by Ian C

A wild cam isn't required for turbo engines (although I've no idea if it is useful in the upper echelons of motorsport like CART and the turbo F1 days etc)

 

Anyway, that's really offtopic now so I'll shut up

 

-Ian

Chasing all the exhaust gases out with a fresh intake charge to the extent that some unburnt air / fuel actually passed right through into the exhaust used to ba called "blowing down". Almost certain that emissions and the quest for more MPG will have killed this off in a road car application though.