Pulse and Glide = no difference?

[JQmile]

It's been about a year since I did this test in my Chevette, but just never got around to posting it up. Got 58mpg on a test loop and was trying to break 60mpg.....so I semi-slowly accelerated to 70mph or so and then would let off till about 50mph, and then speed up. Problem is, my mph would drop like a rock as soon as I let off, and it only took me a few seconds to coast down before I had to accelerate again. I have since found out that the Chevette had a drag coefficient of .45, one of the worst ever for a passenger car...and it was so light, I had no momentum.

So after all my pulsing and gliding, I got.....58mpg. And on a back up test 57mpg. No different. So, bad aero?

[Piwoslaw]

For each car there is a speed above which P&G doesn't work any more. The better the aero, the higher the speed. Most cars with Cd around 0.30 would probably not be P&Ging at 70mph (if they decided to go that fast), so with your Cd you have to SLOW DOWN to see any results. Improving your aero would help, a partial kammback and a smooth underside would do wonders on that car.

Originally Posted by JQmile

I semi-slowly accelerated to 70mph or so

Accelerating semi-slowly is only semi-efficient. You accelerate briskly, keeping the engine at ~80% load and rpms around where you have max torque. That's where the BSFC sweetspot usually is.

Originally Posted by JQmile

and then would let off till about 50mph, and then speed up. Problem is, my mph would drop like a rock as soon as I let off, and it only took me a few seconds to coast down before I had to accelerate again.

The glide part should be done in neutral. If you just let off the gas pedal then you are engine braking - wasting what momentum you have to keep the engine turning.

Originally Posted by JQmile

and it was so light, I had no momentum.

In certain circumstances more weight is better, ie it allows longer glides. But carrying extra weight is more likely to hurt your FE than help, so you should focus on aero and driving style.

[tinduck]

My van has a cd of 0,36 and nearly four square meters of frontal area. Best P&G speed seems to be about 70-80 km/h (50 mph) and the highest speed interval which still gives results (measured with Scangauge) is about 90-100 km/h (60 mph).
This is consistent with the rule of thumb that over 80 km/h, aero begins to dominate the sum of resistances.

Since your cd value is even worse, piwoslaw may be right in advising you to slow down to get better results.

so long,

tinduck

[Vekke]

If P&G does not give better results you are doing it wrong and reasons are many:
- too much speed for cars Cd as it was in this case
- Trying to do it in a "big" uphill. Or even smaller depending on the case
- cars weight
- Cars rolling resistance
- You leave the car to gear
- not best gas position on accelerations
- Power to weight ratio

Yesterday I was educating fuel saving driving techniques in a truck thats total weight was 45 tonn. P&G did work in downhills. downhill was 1 meter down in every 70 meter distance and we already rolled . speed did slow down much in that small hill, but it worked. If it was level or even little uphill it did not work.

On next day we had 50 tonn and it did not work anymore pushes were too long at 80 to 88 km/h speed. We would have need to slow down to see the positive results. However we were still able to coast in neutral on deeper hills.

Also I noticed that your eye lies about 50% of the time in small hills is it downhill or uphill. Only way to tell was to watch height from navigator, very helpful tool in that job! On a bigger heavier and worse aero car or truck it will work on downhills. With better power to weigth ratio car and if aero is good you can do it almost everywhere and see better results.

[Daox]

The problem here isn't the speed, its the technique. To P&G you need to coast with the engine off (best) or in neutral.

[Vekke]

Speed is a problem. If you already have to push about half etc much gas in normal driving you engine already is at good fuel efficiency level in a BSFC map. So there is no room for improvement in fuel consumtion average if glides become shorter than pushes or about the same. Usually if you have to press the gas already hard the engine dont waste the fuel so much and you can predict that car will not coast well in those speeds.

Engine off will help but if the glides are still very short effect can be negative, every time you make a bumb start you brake little. Also its not fun to start a push every five seconds etc.

My advice is that you can test how high speeds you will see better results if you accelerate at level road from 30 to 80 MPH and start coasting in neutral. If lose that each MPH faster than you have gained in acceleration it will not work well. As soon as times start to be so that you can coast so that it take about 1.5 times longer to slow down than it takes to accelerate you start to see some real gains. Best to take it to video and watch it in slow motion etc to be able to tell for sure.

In a downhills it can be done at little higher speed and uphills much slower, if at all.

[Daox]

Sorry, you're right Vekke. I overestimated the horsepower output of the Chevette engine. With 51 hp and the .45 Cd I'm sure you're already running fairly well loaded at 70 mph.

[jakobnev]

The slope of the rear hatch looks about as bad as it gets on the Chevette:



A camm-back may be called for.

[Vekke]

Originally Posted by Daox

Sorry, you're right Vekke. I overestimated the horsepower output of the Chevette engine. With 51 hp and the .45 Cd I'm sure you're already running fairly well loaded at 70 mph.

No problem. I did not even check the power . I would think even thought you would have had 200 hp the gain would be minimal if you make the pushes from 50 to 70 mph. Speed will slow down very fast from 70 to 60 with cd 0.45, even my lupo does. The little gains from 60 to 50 mph would have been lost many times in 70 to 60 drop.

I havent even tried yet with my lupo to P&G from 55 to 67 mph to get the 62 mph average because I know it will just disappoint me. There just aint enough power to make the car fly with about 1000 kg and cd about 0.27 at the moment. Need to first take some weight off and chip for more power and make the cd better. After that it will do the same as now from 80 to 95 km/h about 45 to 55 mph what I do most at the moment.

With that 45 tonn truck I noticed very clearly if you try to push in wrong places you will eat the savings you have gained in successful P&G made before. so get a gauge that shows distance from sea level etc.