Hypermiling - fuel efficiency taken too far?

[YorkshireJumbo]

I may have mentioned this before, but I bought a diesel Micra for commuting for an hour each way up the M1 during the rush hour. I initially tried to get there as quickly as (safely) possible, and would get 45-50mpg. After a couple of weeks, I wondered how much faster I was getting there than those who didn't try so hard. So I drove less for time and more for calmness, and was regularly achieving 60mpg+, while arriving less than 3-4 minutes later

On a long motorway journey a while later, I was trundling along the A42/M42 in a steady stream of traffic, and was averaging over well over 80mpg at times. From then on, I would stick religiously to a 70mph maximum, and try not to accelerate too hard uphill, and I found I could almost achieve 80mpg for the whole journey. That became the challenge: try to beat 80mpg. In truth, I never did average 80mpg for a whole journey. I regularly managed it over large parts, but there was always some part of the journey that let me down. The best I got was 79.9mpg. It wasn't too difficult, and the challenge made it less tedious.

I should add that all these measurements were from the in-dash display, but when I tested it over a few runs, it wasn't far out. 70mph indicated turned out to 67-68mph according to sat-nav

[nigelc]

I toyed with the idea of eco-driving a couple of years ago. My commute to work covers 35 miles of A1. The theory is: On a clear road 70mph = 30 minutes, 60 mph = 35 minutes. The bits at either end were dependant on road conditions and traffic density so were out of my control. Attempting 70mph average is not reasonable as you get stuck behind lorries battling at 55 & 55.5 mph and this will have a greater effect on the average speed at 70mph than at 60mph so the realistic difference in time is only 2 to 3 minutes (not time to make a cup of tea ).

I tried the runs several times with different fuels because I was interested if there really was a difference between premium brands and supermarket fuel. I also tried a fuel additive, Millers Eco-Max. The car was an Alfa 147 diesel.

The results:

Very steady drive (trying to hold 60mph) averages:
Supermarket 58.7 mpg
Shell (pre fuelsave) 61.8 mpg
Shell fuelsave 62.4 mpg
Shell Vpower 64.6 mpg
Shell fuelsave with a dollop of Millers EcoMax 64.6 mpg

Best 3 were 70.0 mpg (Vpower but a following wind (weather not baked bean related)),
69.6 mpg (Shell fuelsave + Millers EcoMax)
68.4 mpg (Shell Vpower)

Normal driving:
Supermarket 46.2 mpg
Shell (pre fuelsave) 48.6 mpg
Shell fuelsave 50.4 mpg
Shell Vpower 51.9 mpg
Shell fuelsave + EcoMax 52.2 mpg

Other noticeable differences were very apparent when giving a hard acceleration after a steady run. Vpower and Fuelsave with EcoMax would produce a wisp of off-white smoke in the rearview mirror first time, nothing afterwards. Fuelsave gives a thin black cloud the first heavy right foot. Early Shell would give a good black cloud first time and maybe a thin cloud the second. The supermarket fuel was a case of “Who took the scenery away?” and would take three good right foot applications to blow the soot out. The engine was noticably quieter and smoother with V-power (or fuel-save with Millers).

Conclusion: Two minutes longer journey saves a lot of fuel. There is a lot of difference between fuels.

[YorkshireJumbo]

Very similar to my experience, but the difference between fuels was very interesting. Didn't think of doing that at the time. Black smoke (aka soot) in the exhaust points to poor combustion, which also means poorer fuel economy. I'm sure cheapo fuels don't have the extra ingredients to lubricate the high pressure fuel pump and injectors, either.

As a matter of interest, did you calculate the cost per mile (or whatever) between the various fuels?

[nigelc]

As a matter of interest, did you calculate the cost per mile (or whatever) between the various fuels?

Didn't actually do that unfortunately. Maybe next time.

[WS]

whether it is indeed best to accelerate gently (for a longer period) or briskly (for a shorter period)? I don't think anyone's come up with a definitive recommendation yet.

The views on this topic are so different because the difference between the two techniques is negligible, at least in terms of fuel consumption (if there is any). As I wrote in the other thread, even if you use precise measurement equipment, it is very difficult to find any differences between the two styles.

The influence of accelerating technique on overall fuel consumption is negligible in itself, unless you drive a heavy vehicle and have to stop and start often - e.g. driving a delivery vehicle in dense urban traffic. In non-urban traffic the acceleration phases are so rare that the way you accelerate has little effect on fuel consumption. There are many things which are more important.

Conclusion (at least for me): accelerate in a way that is rational in given circumstances. If there are no other road users who might restrict the way you accelerate, do it in a way that is more fun . In a hypermiling contest it may make a difference if your fuel consumption average is 0.1l/100 km lower than someone else's, but in real life it is irrelevant. In my opinion.

[Kevin]

The influence of accelerating technique on overall fuel consumption is negligible in itself, unless you drive a heavy vehicle and have to stop and start often - e.g. driving a delivery vehicle in dense urban traffic. In non-urban traffic the acceleration phases are so rare that the way you accelerate has little effect on fuel consumption. There are many things which are more important.

Conclusion (at least for me): accelerate in a way that is rational in given circumstances. If there are no other road users who might restrict the way you accelerate, do it in a way that is more fun . In a hypermiling contest it may make a difference if your fuel consumption average is 0.1l/100 km lower than someone else's, but in real life it is irrelevant. In my opinion.

The fact that acceleration technique makes a negligible difference to fuel consumption seems a fairly logical conclusion and I'm grateful to you for confirming that this has been shown to be the case.

I still get people advocating gentle acceleration, with no supporting evidence, as a means of improving fuel consumption. I feel the emphasis should be on more productive methods.

[Gareth]

I still get people advocating gentle acceleration, with no supporting evidence, as a means of improving fuel consumption.

If people are willing to take longer to get to their destination (gentle acceleration) then they should also consider using a lower top speed. They should be clear about the trade-off, though.