Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Tech question: why do intake valves need cams?

I was just sitting here with too much time on my hands, and I thought: why do intake valves need to be cammed: couldn’t a check valve work just as well?

I mean, in the 4-cycle scheme, one only wants flow in the “intake” cycle, and that flow is into the cylinder, in the direction of reduced pressure.

So, you need a valve (a) that allows flow, in one direction only, when pressure in the cylinder is lower than the intake manifold, and to seal at all other times.

Well, that’s a description of a check valve.

This wouldn’t work for exhaust–it needs to seal for the “power” stroke and open for “exhaust”–but I don’t see the obvious reason for cam-actuation of the intake.

There has to be a intake valve as sturdy as an exhaust valve. They’re both put under extreme heat and pressure conditions. So the valves that are operated by a camshaft are check valves.


One of the peculiarities of spark ignition ICE’s (Internal Combustion Engines) is that the intake valves are opened before all the pressure in the cylinder has been dissipated. The reason is that you get more power that way. Yes, the exhaust gases tend to dilute the incoming charge of air, but it is more efficient to do it that way.

The intake is opened against pressure, which means it needs a push to do so. If you were to disconnect the cam for the intake valve, the intake would open extremely late in the cycle and very little air would go into the cylinder.

For years (and maybe still) Detroit Diesel made a 2 stroke diesel with no intake valves or cam for them. It had ports in the cylinder wall that were either open or covered by the piston going up and down. But I did get in a heated discussion with my diesel instructor as to whether technically speaking the piston was the intake valve.

Yeah, I thought this up when I was watching a vid about “hit and miss” engines–those early 20th century things with the huge flywheels. They had a cammed exhaust (held open during “miss”) and a spring-loaded intake valve.

I guess the most obvious drawback would be that the force on an uncammed intake valve would be limited to somewhat less than atmospheric pressure, times the surface area of the valve. As RPMs increased, I’m guessing this was insufficient.

As capriracer noted, the intake valve is usually opened at the latter part of the exhaust stroke, when there is still pressure in the cylinder. A check valve would not be able to do this, so power, efficiency, and emissions would suffer. There would also be no power at higher RPM’s, if it would even run at higher RPM’s. It could probably be done, but would not be a very efficient design. Nice thought, though.

A couple of more reasons - that kind of valve would only open under vacuum, creating extra pressure drop, limiting the amount of fuel/air mixture that could get in. And these days getting the maximum power while controlling emissions requires precise control of all the valves, something you can’t get with a flapper. I also imagine the whack at closing would be pretty tough on the valve, compared to the controlled closing that happens with a cam.

Older 2 cycle outboard engines have exactly what your are describing. They can get away with it because the fuel/air mix goes through the valve (Reed valve), then the crankcase before entering the cylinder.

That’s the normal setup for all two strokes, outboards, chainsaws, lawn equipment, and (used to be) motorcycles. Lots of power per pound, simple, but high emissions.

It’s more complex than you think…The intake and exhaust manifolds are “tuned” to a certain extent to use rapidly moving columns of air or exhaust gas to help the engine breath…Piston speed varies greatly and so do the pressures measured at the intake and exhaust ports…If a simple “check valve” was used, controlling it properly would be VERY difficult. The pressures acting on it, which vary greatly from idle (closed throttle) to WOT, from low RPM to high RPM would quickly tear this “check valve” apart…Todays most advanced engines use variable valve timing to deal with these problems and produce engines with an amazingly broad power band…

Put more simply, the MASS of your check valve would have to be controlled by a spring…under certain operating conditions, the forces acting on the check valve would be beyond the ability of the spring to control and engine performance would be greatly degraded. If you beef up the return spring so it CAN control the valve, the engine would not idle because the intake valve would never open in the high vacuum environment when forces acting on it would be very low…

We’re all thinking “within the box”.

Some years back an engineering firm developed a working V8 engine that used rotating shafts with ball valves rather than cam-activated valves with reciprocating valvetrain parts. Some years of engineering and some fancy ceramics technology was used to provide the level of sealing and the heat endurance necessary. The final engine was successfully subjected to all the standard automotive reliability engineering testing; accelerated life tests, heat/vibration tests, etc. I don’t know what finally became of the project, but I liked the idea. Energy is wasted opening valves, and valves create restrictions in the flow in and out of the cylinders, and constantly-rotating ball valves seem an excellent alternative.

I’m sure there are alternatives to traditional reciprocating valves, just as there were alternatives to using vacuum to control fuel metering. And if one suddenly appears in the marketplace we’ll all say “gee, that’s such an obvious solution”.

Perhaps there is a gain to be had but what is wrong with having an intake valve? Change is good and all but change just for changes sake is pointless, what will I gain by eliminating the intake valve? It is not like they are high maintenance

The cams are engineered to open and close valves according to the timed intake and exhaust cycles. In the exhaust cycle, the seal needs to be complete because of the pressures involved. This cycle uses the apex of the egg-shape of the cam to close the valve.

The mechanical cam allows no opportunity for error in the cycle aside from mechanical damage. Other systems invite error.

Willis-Knight made a sleeve valve gasoline engined automobile, Mercedes made a straight eight engine with the power taken off in the center of the crank to reduce crank flex without the imbalance of a V-8. It also had valves that were opened and closed by the camshaft- no valve springs. It developed 310 hp from 3 liters in 1955.
Engineers have tried a lot of things to get more power but for engines built with production tolerances, the ones we have today are hard to beat in terms of cost of production,reliability, power and economy.

Early 4 stroke engines had intake valves that opened with the vacuum of the descending piston. Search Fairbanks Morse or Flywheel engines or Hit and Miss. Find a Flywheel Association event near you and go watch the old engines operate.

A lot of antique engines did use intake valves that were simply spring loaded check valves. One of the most unique was probably the Gnome rotary engine used in some WW1 airplanes. This engine had a single exhaust valve in the cylinder head operated by a pushrod and the intake valve was spring loaded and was in the piston crown, the crankcase being used as an intake manifold. The valve had a balance weight to counterbalance it so that the rotating motion of the piston would not hold the valve open.
The exhaust valve stayed open for much of the intake stroke and allowed the piston to pull in fresh air through the open exhaust, the intake mixture was super rich to offset this. This allowed the exhaust valves to run cooler and also a super rich mixture in the crankcase meant a less severe crankcase explosion if the mixture in the crankcase accidentally ignited.
This also meant the engine could not be throttled. If a throttle restricted the intake flow, the fresh air sucked into the open exhast ports would make the mixture super lean and the engine would die.

How would duration changes be made and how would "on the fly’ variable valve timing work out with these systems? I could see a comparable lift issue being easier to deal with (that is a check ball could be made to approximate a desired lift) but duration and making them both variable,I don’t know.

And the crankcase gets pressurized by the downward movement of the piston, so the intake charge enters the cylinder at a slightly higher than atmospherical pressure.