Pumping loss test

[monroe74]

Hi, I'm new here. I'd like to chime in with a comment that's indirectly related.

Recently I read an article about BSFC (http://www.autospeed.com.au/cms/A_110216/article.html). This made me think about pumping losses, so I tried something new. I drove about 350 miles ('95 VX) while trying hard to avoid any throttle setting other than about 70%. This meant I was doing a lot of P&G.

I was pretty stunned at the result: 70 mpg. On other recent fills, I had been doing about 50 mpg.

I'd like to do more testing, and repeat the result, before I decide it's real. On the prior tank, I had overfilled, somewhat, so that might be a factor. But it sure gives the impression that pumping losses are a big deal.

The result is especially surprising on my car, because the large throttle opening pretty much means that I'm spending little or no time in lean-burn mode. So my initial conclusion is that minimizing pumping losses is a more powerful strategy than relying on lean burn.

I have a feeling that P&G is so effective for some people because they are avoiding small throttle settings, and therefore minimizing pumping losses.

Apologies if I'm repeating things that are already obvious to you folks!

[JohnnyGrey]

Yes, P&G will net you greater results than cruising, lean burn or not. That's because you're dumping most of your fuel in a good BSFC region at aerodynamically efficient speeds. When you can no longer sustain those speeds, you lay off the gas and repeat.

[monroe74]

"you're dumping most of your fuel in a good BSFC region"

Right, makes sense. I'm just surprised that the effect is so large on a car that has lean-burn. I figured lean-burn would minimize or offset the normal pumping-losses penalty you get with most cars, when using a small throttle setting.

It seems to me that the importance of sticking with large throttle openings is largely overlooked. For example, DWL seems to be all about maintaining a low throttle setting. So my hunch is that DWL is not efficient, except to the extent that it's simply another way to say "go slow."

[DifferentPointofView]

yes, but Hammering it up hills is way less efficient than coasting up it and getting back to 55 going down. DWL

[monroe74]

Oops, my mistake. I just figured out this got moved to another thread.

[blownb310]

I used the search feature for "negative work" and found this thread. I read this entire thread with interest and I am left wondering what the best combination is.

Do we now have a consensus then, that the best way to go is to use about 70% throttle during P&G on flat terrain to reduce negative work, but to use DWL to reduce fuel consumtion up hills?

If not, please share your thoughts.

Mike

[theunchosen]

I dunno if anyone else mentioned this. . .but if I turn the engine off while driving and leave it in gear it will bump start itself. . .So the only meaningful data you could get would be the split second before the engine rolls over. . .

[theunchosen]

This is also why everyone makes a pretty ridiculously big deal about throttleless engines. . .If it has not throttle the next restrictions are pipe width and bends. Those restrictions are usually small so you go from inducing huge load to draw air in to allowing free flow.

One of the 2 big reasons diesels stomp gassers.

The other is higher compression due to direct injection and now SFI.