No camshaft


#1

I am a docent at an aviation museum and auto museum. One of my fellow volunteers showed me an unusual aircraft engine. It has no camshaft or rocker arms to open the intake valves. It only has a weak valve spring.

On the intake stroke, the suction of the descending piston is enough to open the valve and let the air-fuel mixture in. On the compression, power and exhaust strokes, the pressure in the cylinder keeps the intake valve closed.

I have never seen this on an auto engine. I can see how it could save a bit of weight which is critical on an aircraft. I suppose it is possible that on the compression stroke, a small amount of mixture could blow back into the intake manifold thereby reducing the volumetric efficiency.

Has anyone ever seen this and why isn’t it used in auto engines?


#2

This must have been an extremely old (pre-1914) aero engine. I don’t think that type of valves has been used since then. It offers very poor control of the valve timing, something modern engines require.


#3

Aircraft engines run in a narrow range of speed and (near full) power.
So control of valve timing can be fudged.
Just like car racing engines way back when could run with fixed ignition timing and no thermostat.


#4

Automatic or atmosperic intake valves were used on lots of antique engines, especially the old one cylinder “hit or miss” engines used on old farm machinery. The exhaust valve still needed a cam though.
The Gnome rotary aircraft engine used in WWI only had an exhaust valve in the cylinder head. The spring loaded intake valve was in the piston crown and the crankcase was the intake manifold. These engines were lubed like two stroke engines with a total loss oiling system.


#5

In modern, high-speed engines, as RPM increases, you quickly lose control of the lightly sprung intake valve…It is slammed down on its seat with considerable force and can bounce. If you increase the spring tension, the valve is very late in opening, destroying power output…

The old “71” series General Motors diesels were 2-stroke valve-less design which used a mechanical supercharger (blower) to clear the cylinders when ALL the ports were open…

When Yamaha perfected the expansion chamber (tuned pipe) and reed valves, 2-stroke engines could be made to deliver fantastic power levels…Emissions and long term reliability were another matter…

Today, it seems to me, the direct injected rotary engine holds great promise which is being overlooked…


#6

This is not all that unusual. Many engines have no intake valves at all…2-stroke specifically. Detroit Diesel 2-strokes have holes in the sides of the cylinder walls. Also, think about the one-lunger engines from the turn of of the 20th century…fire only when needed by;—holding the weakly-srpung intake valve open.


#7

Also, think about the one-lunger engines from the turn of of the 20th century…fire only when needed by;—holding the weakly-srpung intake valve open.

Actually, the governor held the exhaust valve open when the engine wasn’t firing.


#8

Yea, sometimes the governor acted on the exhaust valve, sometimes not…there are infinite ways to skin a cat. Suck Squeeze Bang Blow…not much has changed in the 125 years or so…we’ve been a technologically lazy automotive world. I frankly can’t believe we’re still using the recipro-internal combustion antiques…we ought to do better than that. We all buy gasoline for our cars. The gas produces heat when burned and moves our carcasses down the road. However, notcie how much work we do to just get rid of that heat…radiators, exhaust, brakes, etc. I’d frankly like ot use the heat I bought for something more useful.


#9

The more likely way to get rid of the camshaft is via the camless valvetrains most manufacturers are toying with, where you have electronic actuation of a valve (solenoids or the like) but no mechanical linkage. That gives you fine control which you won’t get in the design you describe, but with the benefits of eliminating timing belts/chains, cams, etc… overall you get a more compact engine with lower losses, but the challenge has simply been reliable and affordable actuation.

You actually can even get rid of the starter motor if you have a camless actuated valvetrain. How? The computer knows where the cylinders are in their motion path - it finds a cylinder just past TDC, opens the valve, injects some gas, closes the valve, and fires the plug, and kicks in the next valve as soon as it senses the crank rotating enough to fire that cylinder. The combustion is a bit dirty on the first two or three fires, but starts it up good enough…