Hello all. I have a few questions If you don't mind.


1) how do dual overhead cams make more power than single over head cam on a 4 valve engine?

2) How, exactly, does heat rob power from an engine if it's air cooled?

Thanks for your help,


Jason

 

Your questions invite complex answers; I'll propose a couple of speculative responses.

DOHC. Double overhead cams don't, in themselves, produce more power than a single overhead cam, or even a single pushrod cam. Identical valve timing (lift, duration, overlap) may be available from any of the valvetrain schemes mentioned. The DOHC's advantage lies in its ability performing cam timing with minimal reciprocating mass in the valvetrain. Translation: valves may operate directly off cam followers, in contrast to rockers, for example. With less mass, higher rpm's are possible without valve float. The DOHC advantage thus comes only at higher rpm's.

Heat. Heat, in itself, robs no power. The heat of combustion (burning of the fuel mixture) and friction (rubbing of the internal mechanical parts) must be dissipated, by air flow over metal surfaces with air-cooled engines, or by circulation of coolant with liquid-cooled engines. At extreme heats, when the piston, for example, expands to a point of excess friction with the cylinder bore, we approach catastrophic conditions involving PERMANENT and total power loss. Operating at temperature equilibrium conditions; however, heat is a consequence of power generated and robs no power from either an air- or liquid-cooled engine; except, some power from the liquid-cooled engine may be dedicated to driving the coolant pump.

Just a couple of thoughts, I hope other posters have more; thanks for posing these interesting questions.

Tree Farmer

 

TF: "Heat, in itself, robs no power."

Not quite. I have read that most 2-stroke internal combustion engines convert only 23-25% of gasoline's energy into forward thrust. Arond 33% is lost through the exhaust. The rest (60+ percent) is turned to heat, in effect wasted. So, heat does rob power. Theres just nothing we can do about it.

In theory you can increase operating temperature to cut your heat losses. Usually this works against you, however. The air loses density, and the motor is more likely to sieze. Sure sounds good on the surface though.

Just thought I'd throw that out.

 

Heat
This poses an interesting question. When the fuel is burned the combustion provides power to the rear wheels, around 25-30 percent according to the above feedback. I'm sure this is fairly close to the actual number. Consider this though, according to the second law of Thermodynamics (conservation of energy)energy is neither created or destroyed. This means what ever energy is input, the exact same amount is output. When you build a fire the fuel (gas, wood, or coal etc.) is consumed. The by-products are heat and carbon and water. These by-products carry the energy of the fire with them. It works exactly the same on a internal combustion engine. The energy is transmitted to the rear wheels exhaust and into heat. So the heat does rob power from your engine, but there is not much you can do about it. Some performance vehicles wrap their exhaust headers with insulation, in hopes to gain a few extra horsepower. This works b/c the energy expelled isn't used to continually heat the exhaust headers. Your not going to find big or noticable gain if you do this on your four wheeler however. Wrapping the headers can cause the engine to run hotter exspecially with a air cooled engine this is NOT desireable. If the metallurgy was available to produce an engine to withstand extreme heat and still run efficiently it would be done. However, metals can not withstand high temps b/c of thermal expansion and several other properties. So yes Heat does rob you engine of horsepower, but NO you can't do much about it.

 

Tree Farmer hit the nail on the head with his description of the valve train differences, but I think that the question could be taken further by noting that it is the 4 valves that create more power.
One of the key ingredients to the performance on an internal combustion engine, is Volumetric Efficiency. In other words, allowing each cylinder to completely fill on the intake stroke, and expel all waste on the exhaust stroke, as easily, and quickly, as possible.
Although vacuum is a word easily used when describing induction, a "vacuum" does not exist.
Before the intake valve(s) opens on the intake stroke, the piston creates a low pressure area, which is lower than the atmospheric pressure outside. Hence, when the valve opens, the air/fuel mixture is actually "pushed in" not sucked in.
100% Volumetric efficiency is very hard to obtain under normally aspirated conditions. The only way to exceed 100%, is by using a Supercharger, or Turbo Charger. In the 60's manufacturers used Ram Air Forced Induction to simulate a Supercharged effect, and it did work.( Hmmmm, Ram Air on a Quad, theres a question.)
Besides the restictions of the snorkel, air filter, carb. etc,... four valves per cylinder is the next way to achieving the best volumetric efficiency, or power, along with a low restriction exhaust.

[This message has been edited by r34 (edited 01-27-2000).]

 

The nascar guys all achieve 105-110% volumetric effiency with naturally aspirated engines. How you would do that with a quad I don't know. I assume there isn't enough intake runner to get it to flow correctly. Has anybody experimented with that stuff on a quad? (moving the carb farther from the head to increase bottom end power)
PKN