'Petrol efficiency experiment; an increase of 20%' blog discussion

wiggers

charlieheard wrote: »

An engine is not equally efficient at all rotational speeds, loads and throttle openings.

That's what I meant by specific consumption, i.e. consumption per unit power generated, which varies across the range of rpm and is a minimum at maximum torque and I believe around 75% throttle.

Air resistance is present at all speeds and increases with the cube, so average speed is important. A 10% reduction in speed is a 27% reduction in air resistance. Rolling resistance is more linear and drivetrain friction is more dependant on engine speed, so agreed they are a larger proportion of total friction at lower speeds. But braking does not use fuel, the heat that appears in the pads and disks comes from the momentum of the vehicle. Unnecessary braking means you either had too much momentum (acquired from fuel) or you will have to use fuel to restore the momentum you had before you braked.

The key things for driving efficiently are:

1. Minimise the mass and air resistance of the vehicle,
2. Keep the engine rpm close to maximum torque, (using gears)
3. Maintain momentum, avoid unnecessary slowing/braking, (includes using cruise control)
4. Accelerate according to road/traffic conditions to achieve (3) and bearing in mind (2).

Mark

wiggers

harryhound wrote: »

Would I be right in thinking a hybrid car uses the need to slow down to recharge its batteries ?

That's pretty close. Hybrids use both braking (foot pedal) and engine braking (slack throttle) to generate electricity from momentum that would otherwise be lost as heat. The electricity is stored for later use by an electric motor, saving fuel that would have be used in the engine. Generating, storage and reuse all involve losses, so it isn't quite a good as it sounds. Hybrids are at their best in stop/start driving around town, but can be worse than conventional cars on long journeys at constant speeds.

Mark

Dustsucks

In energy terms, smooth acceleration and letting the engine do the braking will outperform heavy acceleration and braking, all other variables such as speed, remaining consistent between the two scenarios. Heavy use of the accelerator requires a richer fuel mix than normal for various reasons. This is commonly provided by an accelerator pump which injects neat fuel every time you press the accelerator. The result is more fuel required for a given mileage. Also, when braking, energy is lost from the system as heat so any use of the brakes will increase fuel consumption.

I have experimented lately with consumption on a long drive, (Citroen C4 diesel, with cruise control), mostly motorway, from Devon to Chesterfield. On the outward journey with all motorway covered at 67mph, I had a consumption of 60mpg. On the return, with the same load and speed etc., I could only get 55mpg. I can only assume this was due to a slight prevailing SW wind since even if this was only 5mph, it would effectively lower my airspeed to 62mph on the outward trip and raise it to 72mph on the return. From past experiments I know that at speeds above 70mph, economy really starts to suffer. So, perhaps we should take wind speed and direction into consideration and/or average values over out and return journeys when taken over relatively short periods so that outside conditions are unlikely to have changed much.

wiggers

Dustsucks wrote: »

Heavy use of the accelerator requires a richer fuel mix than normal for various reasons. This is commonly provided by an accelerator pump which injects neat fuel every time you press the accelerator.

This was true with carburettor fuel systems but is no longer true with electronic fuel injection/engine management. The latter always provides just enough fuel for the throttle setting selected and I understand 75% throttle is about the most efficient. The reason carburettors were phased out was because of this waste of fuel and the resulting unburnt hydrocarbons in the exhaust, which couldn't meet emission regulations.

Dustsucks wrote: »

Also, when braking, energy is lost from the system as heat so any use of the brakes will increase fuel consumption.

As I keep saying, this is another myth. Fuel is used to increase speed and overcome friction from the engine, air and tyres. Putting your foot on the brake does not use fuel! Accelerating too much, to too high a speed, so you have to brake more, will increase your consumption.

Mark

wiggers

wiggers wrote: »

The difference in total fuel consumption in accelerating gently comes from the lower average speed over a fixed distance (mentioned above)

Just to back this up, I've done some calculations. If you take a 500m stretch between lights and assume you brake and accelerate at the same rate to 50km/h (30mph) then this is what I found. For 1m/s^2 the average speed is 36km/h and for 0.5m/s^2 the average speed is 28km/h. In terms of the difference in wind resistance between these speeds, there is a 52% reduction. Remember, though, that this is over a typical urban dash between lights. If you're going to be on the motorway for half an hour then it doesn't make any significant difference how hard you accelerate or brake. Horses for courses...

Mark

John_Pierpoint

On a motorway it is how fast you go that makes the big difference.
See prior discussion about "slip streaming"

wiggers

John_Pierpoint wrote: »

On a motorway it is how fast you go that makes the big difference.
See prior discussion about "slip streaming"

Not only your average speed but also the extent to which you slow and speed up again. Need to keep a constant speed, using Cruise Control when fitted.

I would never advocate slip-streaming. Too dangerous, unless your car is fitted with a radar system that will brake for you if the car in front slows suddenly.

Mark

charlieheard

wiggers wrote: »

Not only your average speed but also the extent to which you slow and speed up again. Need to keep a constant speed, using Cruise Control when fitted.

I would never advocate slip-streaming. Too dangerous, unless your car is fitted with a radar system that will brake for you if the car in front slows suddenly.

Mark

Totally agree about slipstreaming - very dangerous. At motorway speeds, you've gone over 120 feet before your foot even hits the brake - that's why the "2 second rule" is there! But it's not just at high speed. What happens if 2 cars are travelling along a road, one at 30mph, one at 40mph and a child steps out in front? The higher speed car will still be doing over 20mph when the other has stopped.

Cruise control is OK, but you want to avoid having to use your brakes as well. You'll often see a string of cars, with one following too close and continually flashing their brake lights as they fail to anticipate. See how far you can go without using your brakes (safely!) - you'd be surprised. Just keep your distance and look far enough ahead, and you can go for hours, even on a fairly congested road.

frustrateddriver

Over the past few years I have been using an American fuel pill in the tanks of both my car and vans, which I use for business. In my Mitsubishi Shogun Sport I have been managing 15% extra fuel economy (60 litre tank = around £60, saving me £9 per tank, minus the product £1 = saving of £8 per tank)

The best part though, because of how the tablet works, emissions are slashed (manufacturers claim minimum of 75% reduction!)

I get mine from MPG wizard, 20 applications cost around £20, worth a look, they work for me!

charlieheard

frustrateddriver wrote: »

Over the past few years I have been using an American fuel pill in the tanks of both my car and vans, which I use for business. In my Mitsubishi Shogun Sport I have been managing 15% extra fuel economy (60 litre tank = around £60, saving me £9 per tank, minus the product £1 = saving of £8 per tank)

The best part though, because of how the tablet works, emissions are slashed (manufacturers claim minimum of 75% reduction!)

I get mine from MPG wizard, 20 applications cost around £20, worth a look, they work for me!

The only problem with these additives is that there is no independent test that shows they work. An individual case proves nothing - my MPG (in an old, automatic MPV) has varied from 23.1 to 37.8 over the past 6 months, with the high reading coming from Shell, and the low from Total. But you wouldn't say that Shell is necessarily better than Total from those results. It is somewhat similar to the claims made about Super Unleaded: more power and more economy. When Which? magazine tested them back to back, they found some cars actually did worse on the more expensive petrol.

Until they are independently tested, I'd treat any claims on fuel additives with a large pinch of salt. If something could really save everyone 15%, do you not think the press would be making a song and dance about it? And the 75% reduction of "emissions" - I'd like to see their proof.