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Old 11-06-2014, 09:45 AM   #1 (permalink)
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When to P&G vs just drive

Have you ever wondered, as you're cruising up a slight incline weather it would be best to use P&G with a very short coast, or simply just putter up it at the speed limit? I know I have. On my daily commute to work, its primarily downhill, so I use a lot of engine off coasting with some pulses here and there to maintain speed. However, on my way home, the inverse is true. I find myself going up a bunch of slight inclines.

In most cars, P&G gives gigantic gains due to oversized engines. However, its not as hard to max out the power of the 1.0L in the Metro. Thus, it cruises much closer to peak BSFC than most other vehicles with larger engines. This is probably the biggest reason it gets such great mileage (especially despite its less than stellar aerodynamics).

So, when is it best to pulse and glide versus just maintain speed? In this relatively simple exercise, I'll show you how to determine what is best.

First off, you will want a brake specific fuel consumption (BSFC) chart for your engine (there is a list of them in our wiki). I know not everyone has access to their specific engine. But, if you find some similar ones you should be fairly close.

Here is the BSFC chart for a 1.0L G10 Metro engine. I have modified it slightly for our purposes. I added more grid lines so I could better identify exact values, and I also added the red dots to indicate minimum BSFC for each RPM value.





Next up, I threw a whole bunch of numbers from the graph in a speadsheet. For the RPM values listed, I pulled the maximum torque and the torque rating for best BSFC. From those two numbers, I calculated the manifold pressure for best BSFC (best bsfc / max torque * 14.5). These numbers correspond to the MAP values I see on my scangauge when cruising. Alternatively, you can use the % load values (best bsfc / max torque) if your instrumentation calculates engine load correctly (the Metro does not).





We can see the G10 Metro engine really likes about 73% load for best BSFC.

So, for me, I cruise at 45 mph / 2200 rpm for most of my commute. That means if my vacuum is close to 10.5, there is absolutely no benefit from P&Ging. I'm much better off just sticking at that load, gaining or loosing a few MPH and just riding out the hill.

Actually, reviewing the chart a little more shows I could probably be just fine with almost anything from 7.9 to 11.7 (the 283 circle on the chart) and loose less than 5%.


This was an interesting exercise for me. I had been using the rule of thumb of ~80% load for accelerating and cruising. This got me pretty close, but it turns out I should have slightly better results from loading the engine a little less. This also means I should be able to get away with a little less P&G and nobody ever complained about that. I've been using this technique for a day or two now and it seems to be working quite well.

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Old 11-06-2014, 09:54 AM   #2 (permalink)
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I never p&g up a hill, just p up (watching load more than rpm, unless traffic is too tight) and g down.
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Old 11-06-2014, 10:08 AM   #3 (permalink)
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Finally someone is getting around to writing some tips on how best to drive my car!
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Old 11-06-2014, 10:19 AM   #4 (permalink)
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I p&g always. Uphill may give a very short glide, so short I may not bother to eoc. Downhill is a nice long glide. But I'm still working the p&g cycle throughout. As the climb gets steeper, the glide gets shorter and shorter, until the point where there is no glide, and I'm at my target load/tps in a steady climb. I don't adjust that load, just the duration of the pulses and glides.

I don't have a specific BSFC chart for my car, just a "feel" for it based on the progress of my trip mpg with various approaches.
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Old 11-06-2014, 11:22 AM   #5 (permalink)
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When BSFC charts are made, what is taken into account? Is this at the flywheel? Accessories attached?

I would imagine that, in practice, EoC probably provides a little more gain than can be predicted by these charts, depending on how they're made.

That said, I don't EoC in my Insight as I figure the savings, in absolute terms, are probably peanuts compared with driving at 80% load in lean burn.
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Old 11-06-2014, 11:44 AM   #6 (permalink)
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At 10 inches of vacuum steady, try pulsing at 2-3 inches and gliding as much as you can. Super shallow grades can allow you to store energy at higher bsfc. You get more power from the same amount of fuel at 3 inches, maybe even up to 5.

On sustained uphills but very shallow grades, you might even pulse the flat spots and use inertia as much as you can on the uphills.

If you are using engine on pulses, then go a little higher on your peak speed which increases your mileage in the glide.

regards
mech
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Old 11-06-2014, 01:45 PM   #7 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Finally someone is getting around to writing some tips on how best to drive my car!
If only someone had been in a position to do that before Daox bought his!

I do not see a chart for the HX.
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Old 11-06-2014, 01:45 PM   #8 (permalink)
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Quote:
Originally Posted by PaleMelanesian View Post
I p&g always. Uphill may give a very short glide, so short I may not bother to eoc. Downhill is a nice long glide. But I'm still working the p&g cycle throughout. As the climb gets steeper, the glide gets shorter and shorter, until the point where there is no glide, and I'm at my target load/tps in a steady climb. I don't adjust that load, just the duration of the pulses and glides.
That's exactly what I do, except dfco uphills because there's not enough time to keep shifting, and steep downhills, and eonc on the flat and moderate downhills. I'm planning to use a kill switch and eofc my next trip and see what happens. I still compare dwl up hills sometimes and feel that p&dfco gives me better results, keeping in mind my civic has a relatively high rpm.
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Old 11-06-2014, 02:10 PM   #9 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Finally someone is getting around to writing some tips on how best to drive my car!
Haha, funny.

This does make me want to pickup a vacuum gauge though. The SG does an okay job of showing the manifold pressure, but it does jump around a lot and its harder to truely know what the engine load is. An analog gauge would be great.
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Old 11-06-2014, 02:13 PM   #10 (permalink)
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Quote:
Originally Posted by PaleMelanesian View Post
I p&g always. Uphill may give a very short glide, so short I may not bother to eoc. Downhill is a nice long glide. But I'm still working the p&g cycle throughout. As the climb gets steeper, the glide gets shorter and shorter, until the point where there is no glide, and I'm at my target load/tps in a steady climb. I don't adjust that load, just the duration of the pulses and glides.

I don't have a specific BSFC chart for my car, just a "feel" for it based on the progress of my trip mpg with various approaches.
That is how I used to do it too. If you ever drive the 1.0L, things are quite a bit different. I just got back from a lunch meeting and on my highway (55 mph) trip home with some headwind, I had that MAP pegged at 10.5 pretty much the whole way. I let my speed fluctuate and I never got above 60 mph. I only got to coast a few places in the ~10 miles or so. Once I got off the highway, things were quite different. On my trip there things were quite different and I was P&Ging quite a bit.

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