"Renault unveils tiny two stroke Diesel engine" Wait, what? I'm confused...

[gv280z]

I've always been under the assumption that all diesels were naturally two stroke motors...right? The intake and exhaust strokes are one in the same leaving you with compression and power...that's it, 1st) Extreme compression of O2 and fuel oil injected...Ignition and 2) Power, piston strokes down, hot gases evacuate through the port in the cylinder wall, as fresh O2 is drawn in, piston strokes up compressing....

 

Am I missing something? I guess it's possible that a diesel engine could have it's own normal set of exhaust valves in the head sharing combustion chamber space with intake valves but that just sounds weird to me, a 4 stroke diesel? Intake O2, compression fuel injected BTDC, power and exhaust. You ever heard of a twin cam / dual overhead cam Diesel engine?

 

I thought the 2stroke diesel was one of the major power aspects held over gas engines. What do you guys think?

 

http://www.foxnews.com/leisure/2014/12/12/renault-developing-tiny-two-cylinder-two-stroke-diesel/?intcmp=HPBucket

[Slartibartfast]

Most car or truck engines, gas or diesel, are four-cycle. The fuel delivery is different, but regardless of the fuel, the engine still needs to get the exhaust gas out and the fresh air in, and it uses piston strokes to do that. Each piston has to push out exhaust gas, pull in fresh air or air/fuel mix, and then compress it before it gets to its power stroke. It seems to work fairly well (given that just about everything uses it), but each individual piston only contributes power for one stroke out of every four.

 

A two-stroke motor only has the compression and power strokes, which means it fires every other stroke. It needs something to replace the missing intake and exhaust strokes, and that something is forced induction.

Gas two-strokes like chainsaws use the crank case as an air pump. The piston going down under power pressurizes the air/fuel mix in the crank case, which then shoots into the combustion chamber somewhere near bottom dead center through a hole in the cylinder wall, purging out the exhaust gas and filling the chamber with a fresh charge. It's a pretty slick design, except that some of the fuel/air mix that blows in tends to follow the exhaust out of the cylinder, and they usually rely on oil mixed in with the gas to keep the moving parts happy. Not exactly clean-burning, but great power-to-weight.

 

Two-stroke diesels use external superchargers and turbochargers instead. The crankcase has oil in it and the bearings have a proper lubrication setup like a normal motor. If some of the intake charge blows into the exhaust, so what? It's a diesel, so the intake charge is just air. They're usually used for big stuff like powering ships or gensets, but it looks like Renault found a way to scale it down.

 

This is all from past and present Wikipedia binges, of course...

As to why there aren't a lot of diesel twin-cam setups, well, the motor in the story has its horsepower and torque ratings at 1500 rpm. I imagine the advantages of overhead cams and other stuff developed for higher-revving gas motors don't matter much when your tach only goes up to three grand.