Displacement on Demand in a diesel?

[jmabarone]

I want to open up with saying that I've been surfing this forum for several months now and this place is really great. I'm 17, and I am but an aspiring engineer. My passion is in motorsports, but that is just a bit of background.

anyways, I was thinking about diesel power as a great alternative to gasoline these days (see Audi R10), and I started to wonder about displacement on demand in a diesel. Considering that diesels get the same fuel economy no matter what the load, wouldn't it seem logical to be able to just do what GM is doing, but on a diesel? I figured that a V8 into a V4 would probably not be the best basis, but a V12 to a V6 would seem feasible in say, a Ford F350.
I was just wondering what some more experienced and learned individuals would say to my ponderings.
thanks,
Jake

 

[MikeHalloran]

<<< aspiring engineer. >>> Do post again when you graduate. It's against the rules now.

<<< motorsports, >>> Great, but get an education that's not so narrowly focused. There's a lot of cool stuff for you to work on. Some of it hasn't been invented yet.

<<< displacement on demand in a diesel >>> In a very real way, that's what turbochargers do. Of course there are other possibilities.

<<< diesels get the same fuel economy no matter what the load >>> Uh, no. Not true for any engine known to me. Maybe you misunderstood something you read. E.g., Diesels without turbochargers flow the same amount of air at any given crank speed, regardless of load. If you don't increase the fuel flow as the load increases, the engine slows down.

<<< do what GM is doing >>> Working hard at going out of business?

<<< I figured that a V8 into a V4 would probably not be the best basis, but a V12 to a V6 would seem feasible >>> Oh, you mean cylinder deactivation. It works on V8s, too.

As for Diesels, it's been done already. I rode on a boat with V10 Diesels that run on five cylinders below 1400 rpm... and shake the hell out of the boat. The late owner had special injection pumps made so they would run on all cylinders all the time. They were real smooth, but smoked something awful. See, just deactivating the cylinders is the _easy_ part of the problem.

<<< more experienced and learned >>> I've been breaking stuff for four decades after graduation. Pretty soon I'll be good enough to take lessons.



Mike Halloran
Pembroke Pines, FL, USA

 

[BrianPetersen]

Displacement on demand with a gasoline engine finds its energy savings largely by reducing "pumping losses" in the cylinders that are switched off. With a diesel, you don't have pumping losses due to closed throttles because the intake side is unrestricted (Note: on newer ones there is often a "throttle" to allow the EGR system to operate properly, but it doesn't generate a vacuum behind it like the way a gasoline engine throttle does; it just acts as a slight restriction to make the engine suck in exhaust rather than fresh air. On VW P-D diesels, the "throttle" only closes if the EGR valve is full-open and the system detects that it still isn't drawing enough EGR.)

The conventional thinking - which I realize isn't always right - is that there is not much purpose in doing displacement-on-demand with a diesel because you won't end up saving much, if anything. The frictional losses of moving the pistons up and down will all still be there no matter if the cylinders are active or switched off. Diesels use a lot less fuel at idle (and light load) than a same-displacement gasoline engine anyway.

Hybrid powertrains switch off the engine completely when not needed - that's a different story; it gets rid of ALL the friction and pumping losses when the engine isn't running. But, a 4-cyl VW TDI diesel only uses about 0.5 litre of fuel per hour at idle as long as the air conditioning is off; that's what you've got to work with.