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Yes, and it's not a simple answer Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Wed, 18 Aug 2004 05:30:02
In Reply to: Are there any benefits to having 5 valves per cylinder?, J, Tue, 17 Aug 2004 07:04:12 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Not that I ever provide one.
Yes, the major advantage of 5 valves per cylinder is that it CAN allow better breathing. AND with today's variable valve-timing engines, it gives you more flexibility in terms of how you set up that breathing.
Here's the complexity part - just increasing the number of valves/valve area does not improve performance. If I remove a 1.5 liter Kia engine and replace it with a 455 big-block, I'll get no more performance if they're both breathing through a straw.
Item 1- engine performance isn't just horsepower at high revs. Most designers look for a broad power band, which usually means a flat torque curve. That gives the best drivability. High-strung, peaky engines are great for racing, not so good on the street. So the designers design the performance for the application.
Item 2- the engine is just one part of the power generation/distribution system. Designing a car is a hellaciously complex task, consisting of many trade-offs. The intake and exhaust systems must be matched to the engine breathing, or you get less power and lousy driveability. You need to be able to package the thing under the hood, and good airflow doesn't like lots of turns. You've got to make it affordable and reliable. You've got to design it for multiple markets with different fuels, so flexibility is important. Oh, yeah, you've got to make the components (head, valves) produceable without lots of cost, and easy to assemble, or the engine will be too expensive. Oh, yeah, don't forget emissions.
These are just a few of the design issues to deal with. If designers could get away with no-valve two-strokes to meet the need, they would. OK, maybe not some Italian cars. Extra valves cost money and weight and noise. Designers don't add them unless there is a value.
So to solve a particular set of requirements, designers may need to go to 5 valves per cylinder in order to get the horsepower/torque required given the allowed runner length underhood. Maybe they need to go after a particular emission limit they're bumping into, and with an extra valve and cam, they can knock down the NOx in some particular area. Maybe it's a better approach than variable inlet geometry.
There are PHd's in Aerosol Physics (yes, it does exist, I know one) working at car companies that know more about boundary air flow at the valve opening and cylinder swirl than any of us even know exists. Given the hundreds of millions of dollars it takes to develop an engine, this isn't just a few guys sitting around with drawing boards.
So at the very simple level, more valves means better air breathing. However, more valves isn't always the best solution, and may not even be added to improve performance, but to get around some other engineering problem.
posted by 192.249....
, Wed, 18 Aug 2004 14:02:33, Wed, 18 Aug 2004 05:30:02 <-- Viewing This Message
, Tue, 17 Aug 2004 18:54:44
, Tue, 17 Aug 2004 16:13:09
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