Back pressure exhaust and engine temp

[gear_dawg]

Is it important to have a certain amount of back pressure exhaust on the valves to keep the engine at a "correct" operating tempature? Is it ok to "overvent" the exhaust? I have this question because a friend of mine told me that it is important to have "backpressure" on the valves so that your engine won't run overcooled. This would be for a consumer vehicle, and how would that affect engines in F1?

Thanks in advance,
Gear

[Ted68]

Try this thread

viewtopic.php?t=3098

[Tom]

I've heard the term 'backpressure' before, some idiot started quoting his limited knowledge of it when I had a hole in my exhaust, summing up with 'it will affect the horse power and eventually blow your engine.' complete crap of course, the car soldiered on a long time before I weilded it up.

It is far more complicated than most car designers worry about but the F1 guys will try to have a certain length and shape of exhaust to make the use of several factors. I'd be a hipocrite to start going on about it when I honestly don't really know, but the vauge idea, I half picked up from an engineer was that gasses exiting the exhaust send shockwaves back down the pipe and the perfect length pipe will have the shockwave arriving back at the valves just in time when the valve opens. I can't remember the reason.

I certainly don't remember overheating mentioned, hopefully someone will clarify as I've now forgotten where I was going. I think perhaps you are not sure what backpressure is so you wouldn't understand the answer to the question you asked. That sounds offensive but i know I wouldn't get it.

[zac510]

It's complete rubbish, as Tom said.

The thermostat controls your engine temperature, not the exhaust.

[bhall]

Back pressure mainly affects exhaust temperature and flow rate, which I guess could indirectly affect overall engine temperature to some degree.

The main thing about back pressure is engine performance. Road cars have to be designed with a correct balance of back pressure and free flowing exhaust to perform at an optimal level.

Tom's right though; it's really complicated. I only have an elementary understanding of the subject.

Basically, too much back pressure makes it more difficult for the engine to run efficiently (imagine breathing through a straw). But at the same time, too little back pressure can be just as bad (scavenging, loss of low-end torque).

The trick is to find a way to keep exhaust gases flowing at a good speed, depending on the application. Most, if not all, road cars need some back pressure to in order to keep the engine running well. The level of back pressure should at least be high enough to keep the exhaust system from scavenging raw fuel and fresh air from the combustion chamber, and to keep exhaust temperatures and velocity at a certain level (again, depending on application).

In F1 cars, I'm not so sure that back pressure is a huge concern, as I think an F1 engineer would want as little back pressure as possible. The only performance area that would be affected by a completely free-flowing exhaust would be the low-end torque, which an F1 car already lacks. At high RPM, a freely flowing exhaust system will help an engine's performance. Modifications to an engine can be made to eliminate scavenging and other similar problems a road car would have if left with completely free flowing exhaust.

But, I don't think you'd want to "overvent" your exhaust, if by "overventing" you mean cooling. If your exhaust is cooled too much, the velocity of the exhaust gases leaving the engine will be slowed down causing more back pressure and robbing your engine of performance and efficiency.

[theSuit]

There will be resonance going on in the pipe - just like a musical instrument - determined by the length and the speed of sound. The position of nodes and anti-nodes in the pipe will effect how easily the spend charge can be ejected, so will influence power, efficiency and (to some extent) the amount of cooling required.

Of course getting resonance to occur depends on the system being excited at the right frequency - so in normal words - for a given length of exhaust you only get true resonance at one engine speed. Unless you can change the length of the exhaust to match - oh no, that's illegal in F1.

And there are similar things going on on the inlet side. Well, not any more - they've banned variable inlet trumpets.

[DaveKillens]

Too much backpressure inhibits the free flow of gases, which is contrary to engine performance. The idea is to get as much air in and out of the combustion chamber as easiy as possible. But you never see any decent engine without some form of exhaust, and it's for a reason. If that wasn't true, then we would see no exhaust pipes at all coming from the cylinder head. Having a certain length of exhaust tube, if properly sized in diameter and length, assists the evacuation of the exhaust chamber. And the hot exhaust gases have to be moved away from the engine and anything else that could suffer from such heat.
So the idea in high peformance engines is to have as low backpressure as possible. In fact, a properly designed collector system does produce negative pressure in cylinders waiting for the exhaust valve to open and allow the gases to flow out. The only problem I see is that if the exhaust gases leave the combustion chamber in an uncontrolled manner, they could melt things such as exhaust valves.

[joselu43]

That is right. It is all about engine breathing. As Tom said, the shock wave gets to the end of the pipe and reflects back towards the engine as an expansion wave. Behind a shock wave you have high pressure, but behind an expansion wave, you have low pressure. If the expansion wave arrives when the valve opens, the low pressure will help to evacuate the high pressure combustion gasses. The timing is difficult to calculate because we have different lengths to each cylinder, different pipe temperatures, geometries etc.
so fine tuning has to be empirical by nature.

Jl

[DaveKillens]

The exhaust valve opens, hot gas and a pressure wave leave. Behind this pressure wave there is a lower pressure area, which helps to "suck" the hot exhaust gases out of the combustion chamber. The pressure wave travels down the exhaust pipe, comes to the end, and another pressure wave then bounces back up the pipe toward the exhaust pipe. This pressure wave is of positive pressure, and it arrives just before the exhaust valve closes. This pressure wave "forces" the air back into the combustion chamber. The exhaust valve then closes, and the compression begins. Timed properly the engine is "supercharged" by the pressure waves "forcing" gas into the combustion chamber. The more air you can get into the combustion chamber, the more fuel you can burn, and thus make more power each power stroke.
The same trick is used on the intakes for the same reason and effect.
The velocity can be calculated quite accurately, and by constructing pipes of a specific length, they can be used for maximum efficiency at a specific RPM.

[jgredline]

Dave
Your correct and save a lot of typing. I may add that the way a proper exhaust header and pipe is determend is by adding a pressure transducer to the pipe to actually measure back pressure. For example. In a street car about 6psi of pressure in the pipe is pretty good and will result in good overall performance. In a touring car road race engine about 2 psi of back pressure is good so that it will still perform well out of the corners. On a drag race engine 0 psi of pressure is the majic number. If the number goes from psi to a negative number then the pipe is too big causing a loss of overall power.
When a header breaks it causes a negative affect causing a loss of power and very shortly after valves will burn. In a f1 pipe you will find an 02 sensor, and egt sensor and a presure transducer to measure presure in the pipe. Often times this is how they know when its about to blow up. The telemetry back to the pits will say much.

[joselu43]

As far as I know, when a pressure (shock) wave bonces of a wall, it reflects as pressure wave. When a pressure (shock) wave bounces of a free boundary (opening), it reflects as an expansion wave. Your mechanism does not work. Here is why. A pressure wave compresses the flow i.e. the pressure behind it is higher than ahead of it, so when the presure wave gets to the end of the pipe, the pipe is full of high pressure gasses (obviously!). If what comes back up the pipe is a pressure wave, the gas in the pipe would be compressed further and the process would repeat, the pressure in the pipe increasing ad infinitum. What reflects from the opening is, instead, an expansion wave that returns the pressure to the initial value and the whole process repeats itself.
High pressure in the exhaust pipe would hinder the gasses from leaving the cylinder. Low pressure would help. The key is to time the expansion wave arriving when the exhaust valve opens. In your proposed mechanism, you have a shock wave arriving when the exaust valve closes, to help compression, but you woud be pushing hot burn gases into the chamber ... not very desirable. You may be right, but I do not think that is what happens.

Jl

[jgredline]

As far as I know, when a pressure (shock) wave bonces of a wall, it reflects as pressure wave. When a pressure (shock) wave bounces of a free boundary (opening), it reflects as an expansion wave. Your mechanism does not work. Here is why. A pressure wave compresses the flow i.e. the pressure behind it is higher than ahead of it, so when the presure wave gets to the end of the pipe, the pipe is full of high pressure gasses (obviously!). If what comes back up the pipe is a pressure wave, the gas in the pipe would be compressed further and the process would repeat, the pressure in the pipe increasing ad infinitum. What reflects from the opening is, instead, an expansion wave that returns the pressure to the initial value and the whole process repeats itself.
High pressure in the exhaust pipe would hinder the gasses from leaving the cylinder. Low pressure would help. The key is to time the expansion wave arriving when the exhaust valve opens. In your proposed mechanism, you have a shock wave arriving when the admission valve opens, to help compression. You may be right, but I do not think that is what happens.

Jl

your explantation does make some sense. Thats what what many books say and teach. You right in if I am reading it right that valve overlap dictates much of what A pipe should be. My simple answer and explanation was based on two things. 1) it comes from over 25 years of working on Dyno's ( I own three of them) and building world class race winning race engines. 2) I wanted to keep my answer simple so as an average mechanic can understand it.
JG

[Ciro Pabón]

Sorry for all the recycling.

You could say no one in this thread runs bikes. There was a talk on six-stroke engines and this repost may help:

... for example, take a look at how you can use back-pressure in the exhaust chamber to "get rid" of valves in two-stroke engines (piston is at left, exhaust at right, red is burnt gas, blue is fresh air-gas mixture). It is sort of a "Miller-type" tweaking, but without valves and at the exhaust, not at the inlet: I wonder if this old technique is used somehow in F-1 chamber design to help the ever faster valve system.

You have to remember that fuel cools the engine, so I guess you can affect engine temperature by omiting the exhaust pipe, a very important part of your horse-bucket. DaveKillens talked nothing but the truth about expansion chambers on this thread on F1 engine noise, and you may find interesting another re-post, with links to some software:

Expansion chambers are essential for, let's say, "valveless" functioning of two-strokes engines, and it is the first thing you should do to enhance your bike. These are different from mufflers.



I am completely sure that, for example, Ricardo's software (warning! shameless link to my own page ) allows you to optimize the exhaust pipe form to treat the entire column of air as a single aerodynamic flow, from engine inlet to final outlet. Actually, this is what they excel at.

You can download IWT program for expansion chamber design here and you will get something like this:



If you find free software to design your four-stroke engine's exhaust pipe [url=mailto://ciropabon@gmail.com]I would be grateful if you send me a mail[/url].

[DaveKillens]

joselu43, think of the four stroke cycle. At just before top dead center (TDC) on the power stroke, both valves are closed and the spark plug fires, igniting the gas/air mixture. The piston travels past TDC and travels down in it's power stroke. Then near the bottom the exhaust valve opens, and while the piston travels up the exhaust gases are expelled by compression by the piston, the expansion of the gases, and finally, inertia of the exhaust air. While the exhaust is still open, near TDC, the intake valve begins to open and cool, fuel rich fresh air starts to pour into the combustion chamber. This is the "overlap" period, and during this time most of the exhaust has left the combustion chamber, and intake air/fuel has entered it, and even begun to flow out of the exhaust valve. This is the period I was referring to, this brief overlap time. But proper timing and exhaust pipe length will see a positive pressure wave travel back down toward the exhaust valve, cram some of the fresh intake fuel/air mixture back into the combustion chamber just as the exhaust valve closes.

I once dabbled in 2-stroke engines, Ciro, with mixed failures.
I was the wrench for a pro level motocrosser in 1976 +3 years, with a CZ. We did a lot of tinkering, and I regret not patenting some of my ideas. We had got hold of early Girling gas/oil shocks and had modified the rear to give 10 inches of travel. We had built a water-cooled head that did very well, even though the radiator was a heavy steel lump scavenged from an auto heater core. We designed and successfully built a two carburetor setup, although we never solved the high tension in the springs, and it cramped the rider's right forearm terribly after 20 minutes. And we even designed and fabricated our own expansion pipe that sort of follows the path of modern motocrossers. Back then the formula for pipe dimensions was calculated by pen and paper, nothing close to modern CAD.
For three guys with almost zero fundng, we had fun, but never ironed out all the bugs and developed the systems as they deserved. But that bike made a whole bunch of power.

[joselu43]

Dave, I understand what you are saying, but for the fresh mixture to make it all the way to the exaust valve quickly you need a low pressure at the exhaust pipe. The low pressure is created by the expansion wave traveling back up the pipe. For two strokes wave physics are the same but the mechanics are not, the whole thing is far more complex that for four stroke. I do not dare to go there.

Jl