BHP vs Torque

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Captain Muppet

8,540 posts

264 months

Tuesday 24th December 2002
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JonGwynne said:

Captain Muppet said:

JonGwynne said: ...That's why I can't understand the use of poppet valves (i.e. reciprocating movement) to control flow...


Cheap, low wear, good sealing, simple lubrication. What are you proposing as a better solution?

BTW Good answer DM!


Cheap? Surely not. They are mechanically complex and therefore expensive to design and built.

Low wear? OK, I'll give you that. Under normal operating conditions, they don't wear a great deal. However, if your cam-belt breaks...

Good sealing? Compared to what?

Simple lubrication? Really? Again, what are you using as a comparison?

Options? How about ball valves and sleeve valves to name just two?


Complex? It's a pin with a head on it, manufactured with forging, a bit of welding (if you want high temperature resistant heads) and some simple turning. Hell you can make a prototype poppet valve in half an hour on a lathe in your shed.

Yes, they will get smashed if your cam belt breaks, but only if they have been designed to hit the piston at TDC and assuming that your engine is fitted with a cheap unreliable belt rather than a nice sturdy chain.

Good sealing in that the combustion pressure forces the valve against the seat. Compared to say, a sleeve valve.

Simple lubrication in that they just sit there at the bottom of the tappet chest covered in oil - no special drillings in the head.

I'm sure the manufacturers are deliberately avoidng ball valves and sleeve valves because there is a comspiracy against low cost effective solutions. Or maybe it's because the valve manufacturers are blackmailing them...

Miguel

1,030 posts

264 months

Sunday 29th December 2002
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Excellent answer to the original post, Graham Bell, and thanks for the great Keith Duckworth quote. When asked by non car people, I just say that power determines how much performance (top speed and acceleration) there ultimately is, whereas an abundance of torque gives you easy, effortless access to whatever performance the car is capable of.


funkihamsta said: Can anyone explain why you can improve torque by say making the inlet path longer with more bends in the piping etc...



While danger mouse (best name at PH) answered the above question far better than I ever could--and taught me a few things in the process--I believe that something could be added.

As anyone who's either built engines (not me) or read Wizard Vizard's articles or books knows, mixture velocities are extremely important for all-around performance, as that is how cylinders are filled as much as possible (high volumetric efficiency), producing the best overall performance. We also know that reading about it is far easier than doing it. Anyway, getting maximum flow into the cylinders is essential, but what's key here is to do it under the conditions in which the engine will be used most of the time.

In an all-out race car, that could be at full throttle within a range of 1000-2000 RPM at the top of the tach. Short, fat runners work great here since there's enough cross-sectional area to get all the air in, and being short, it doesn't have to go far, so it can happen at 18,000 RPM.

In a family saloon, it's usually at part throttle at, say, 2000-4000--BMW M5 excluded. For a sports car, it'll fall somewhere in between. Runners that are too big in diameter for a street driven car could actually kill its performance under the conditions that the engine experiences most of the time. Here's why:

Mass Flow Rate of a Fluid = Density x Velocity x Area =
D x V x A (cross-sectional)
because I can't figure out how to use the appropriate Greek letter for density with my computer. Assuming a close enough to constant density in a naturally aspirated engine, what happens when a low flow of air has to go through runners of different cross sections? More cross-section means a lower velocity, as can be shown by putting your thumb over the opening of a water hose when little water is going through it. Mathematically, V1 x A1 = V2 x A2, or V1 = V2A2/A1, meaning that air velocity is inversely proportional to the runner's cross-sectional area, given the same flow.

Besides the tuning for the shock waves that DM mentioned, a long, thin runner will get a cylinder filled in a hurry even when the engine's at part throttle at low revs. Plus, its length assures that there's enough volume of air to fill the cylinder that is ready to go in, since it's thin and wouldn't have enough volume otherwise. This doesn't necessarily increase an engine's peak torque--it increases it under specific conditions where it may be desirable, such as what the engine may encounter under most driving conditions.

High RPM power and good low RPM torque and throttle response used to be mutually exclusive. With today's technology, the best of both worlds can be achieved with intakes that contain valves changing the length and inner volume of the plenum or runners, not to mention variable cam timing, etc. Fuel injection makes this type of intake possible, or, at least, far better since their manifolds are dry, thus not having to be compromised by having to transport fuel with the air.

>> Edited by Miguel on Monday 20th January 02:24