12 valves?

Yesterday we went to the sale in Pocahontas and we were watching a little pickup sell. Nissan truck, wouldn't start - either dead battery or brake or neutral safety switch malfunction. Anyway, the engine said it had 12 valves. I was a 4 banger engine, so I'm confused. 2 valves are good for intake and exhaust. What's the third valve for?

(I could see the use of a third valve in the new engines that can shut down half the cylinders for better fuel economy-but on a 4cyl?) Most everything I've worked on says either Case, IH, or GM on it though. This has to be a weird Nissan gimmick.

It's not exactly a new idea either. Back in the 70's the Honda CVCC engines used a 3 valve/cylinder setup. As I recall it had 2 different-sized intake valves. The larger ome admitted a relatively lean main mixture; the other admitted a small volume of a richer mixture. I presume this was done to reduce emissions.

And going back to the late the 1930's the IHC "gas start" diesels also employed a 3 valve arrangement. However on these engines a smaller 3rd valve was used only during starting to open up spark plug chamber and to lower the compression ratio.

My 79 Colt hatchback got very impressive gas mileage and ran real well. But I remember that later I had some carb trouble with it, and it was one of the most complicated carbs I ever worked on.

A few years later, the emission standards got more restrictive, and the manufacturers decided to go to computer controlled electronic fuel injection. I am sure that move was one of the reasons that vehicle prices went up so much, but in my opinion, going to injection was overall a good move.

Dealing with the emissions regulations forced the manufacturers to change their products, which they did in gradual steps. I think they continued to use carburetors on lots of models because it was cheaper, and allowed them to put out a lower priced vehicle. But as complex as the carb on that Colt was, I bet the carb was one of the most expensive components on that car, and probably not all that much less expensive than the later fuel injection.

A buddy of mine had an early 80's Datsun pickup with a 4 cylinder engine that used 8 spark plugs. He never mentioned that it had more than 2 valves per cylinder, but I remember the engine as looking fairly exotic at the time. He claimed to get well over 20 mpg with that pickup in normal driving, which was better mileage than others were getting with their mini-trucks. The other thing I remember about that pickup was that it rusted worse than anything else around other than 73-76 Chevy pickups, which must have been the absolute low in Detroit's rust protection.

Some later vehicles had more than 2 valves per cylinder, like the GM Quad 4, and the 4 valve versions of Ford's 4.6 OHC V8. I think that most of them were given the extra valves so the smaller displacement engines would yield more horsepower. If you check the specifications on most of the 4 cylinder engines currently produced in cars, almost all have 4 valves per cylinder, and give horsepower ratings that exceed what lots of V6's did 10-15 years ago.

Aaron

Two small valves will be better than the largest they can get in the space. Valves have to be opened against a spring (generally) and this takes power. One big valve closing needs strong springs and often double springs. Lighter valves are more easily controlled at higher engine speeds.

Gas flow can be improved with two smaller valves rather than one large one - more area and disribution into the combustion chamber can be improved.

Other things like cooling the valves (exhaust) and valve timing alterations as well as rate of lift can all have benefits.

Hope this helps.

Regards, RAB