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Old 11-06-2014, 10:45 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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AndrzejM (03-09-2015), bespurcell (11-24-2014), Ecky (11-06-2014), JRMichler (11-06-2014), MetroMPG (11-06-2014), MobilOne (05-02-2015), oldtamiyaphile (11-07-2014), Xist (11-06-2014)