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

Dave72_2

charlieheard wrote: »

I couldn't disagree more. Half a tank of fuel weighs around 20kg, compared with a car and driver weighing around 1.25 tons for a small family hatchback. As almillar mentions above, you'll waste more than you save by having to divert to the petrol station twice as often. And there's no point recycling the air in your car - it just causes the windows to fog up. Cars generate too much heat: that's why they have radiators to get rid of it. Heating the air that goes into your car just means that less heat is rejected through the radiator. You won't save on drag because cabin air is taken from a high pressure area where the air's moving slowly anyway.

As you say, all cars are different, but most modern diesels just don't function efficiently below about 1500rpm - you can feel them struggling below this. However, petrol engines operate most efficiently when they're at lowish revs and large throttle openings - less pumping losses in the engine. So making sure the engine isn't struggling around town will do more for the fuel consumption than grinding it along at low revs. Once out of town, by all means get into the highest comfortable gear.

If by "following lorries", you mean slipstreaming them, that is incredibly dangerous. To get the effect, you need to be so close that you wouldn't even have time to react and move your foot to the brake pedal if they stopped suddenly. I wouldn't want to chance hitting the back of a lorry at 50mph however much fuel it might save!

I do agree that keeping your speed down and driving less aggressively saves fuel. I now travel at 70mph max on motorways, trying to keep far enough away from the car in front that I don't need to touch the brake pedal, and it's bumped up my consumption significantly over cruising at higher speeds. Doing this, I managed to get 80mpg out of my Micra diesel on long journeys instead of 55-60mpg. London to York (200 miles) that meant 2.5 gallons instead of 3.6 - a saving of 1/3! I'm sure that 50mph would improve it more, but it would add over an hour to the journey...

As you say all cars are a bit different. My Polo TDI does indeed bog down and wont move much at less than 1,500rpm. I find between 1,500 and 2000rpm is best for economy.
Not sure about your statement that petrols can go better on lower revs though. I test drove a MK1 Yaris 1.0 petrol [manual] and it hardly moved, and the design of the engine (no low down torque whatsoever - not nice!) means it went from no revs to about 3000rpm almost in one go. My old 1.2 petrol [manual] Corsa did however manage better than the Yaris in this respect. The odd thing with the Polo TDI 1.4 is that when slowing down in gear (on low revs), it tends to want to keep going, whereas a small petrol car would simply stall out. Yet pulling away in it, it doesnt like the revs that low. The bigger engined Astra (petrol automatic) I used to have had a nice wide torque band, and that went fine on very low revs, but then it was an automatic.

My air recycler in the Polo doesnt stay on for ever and ever. It switches its self off after about 5mins.
- also, if the fan or A/C is on, the recycler kicks in automatically if I engage Reverse gear - the handbook claims this is to prevent fumes entering the cabin. It reverts to the previous position when taking the car out of reverse gear.

On carrying extra weight of a full tank of fuel. I agree that more trips and stops at the filling station counteract part of the saving made by carrying less fuel. Also for those of us with smaller cars, this can happen quite often with only a 45L fuel tank. The chap at work was saying the same about his wifes new Panda. Even better economy than the Polo TDI, but only a 35L tank, so the needle drops down quicker than you expect from a car with such high economy, thus more trips and stops/starts at the filling station.

The anticcipation point is very valid. You want to avoid getting too close to the car in front - leave a big air gap. One, this should mean better aerodynamics, and Two, if the car in front keeps touching the brake pedal for the slightest bend or downhill stretch, you wont need too. With fuel prices going higher and higher recently, I have been doing this a lot.

The best I have managed so far from a single tank of diesel is 546miles. Over Xmas I used the polo to take people to and from home in town, and those short trips (2miles each way max), in cold weather used a load more fuel than is normal for my car. I thought I had a fuel leak! When the weather got warmer again, and returning to the longer trips to and from work, the economy soon returned to normal. Used it one weekend recently to do a longer trip, Chippenham to Warminster, and staying at 50mph/no more than 2000rpm, I got amayzing economy. The needle hardly moved

One last thing, I thought the official mpg combined figure for my polo (TDI 1.4 70PS) was about 50, but the parkers web site states 62mpg (though this doesnt state weather its combined cycle or not). In a way I hope its closer to 50mpg - I cant justify changing it so easily if its up at 62mpg. The cars I am looking at have between 47 and 53mpg, but on a bigger diesel engine.

And one more thing, anyone looked into recent values of 2nd hand cars. I know we had a rise in duty/vat, but with the way running costs are right now, is this slowing the depreciation of the most economical cars by any chance?

Decland24

Having followed Martin's tips, right down to switching the engine off at traffic lights, my daily 25 mile each way commute to work on the M25 uses less fuel than the manufacturers' claims in the owners handbook, which works out to be about 10 - 15 mpg more, or 75 miles added to the range. I sit on the inside lane, stress free, doing about 60 mph, arriving at work on time. And thanks to Nectar and a Nectar credit card I accrue enough points each month to spend them on something needed. This month it was a water filter with 9 months supply of filters.

Idiophreak

almillar wrote: »

Mushypeaman - you're doing it wrong. Drive progressively, drive to go, not to stop. If the lights are green, keep going (unless you KNOW they'll change), if the roundabout is clear, it's more economical to slow down less, as it means you'll have to accelerate less on the other side. Don't hold up queues of traffic! Try only doing all the economy stuff when there is no-one behind.

I wonder how this "economy" balances out with tyre wear, though...Clearly maintaining speed more on roundabouts, through turns etc puts more stress into your tyres - so I wonder where the cut-off is when it's more sensible to slow down a little and save your tyres than to keep your speed up...

almillar

Idiophreak - this depends on how much you spend on tyres, and of course the price of fuel. Tyres don't really wear that fast if you're not screeching round the corners, so as long as you're not going mad, with the ever increasing cost of fuel, less slowing down and speeding up is better for me. You haven't taken into account less wear on the brake pads, tyres, clutch, engine etc from slowing down and speeding up too...

Dave72_2

not forgetting to block change down through the gears when slowing to a stop. Stay in 5th gear as long as poss, then press the clutch pedal, and get ready to select the next gear - eg 2nd if the junction is clear. No need to go, 5 - 4 - 3 - 2 - 1.

Surf_Angel

I having only recently realised how much money I can save driving more efficiently. I have found an app for my iphone that really helps. I put in the distance I am going to travel and it shows me how much it will cost based on different fuel efficiency worked (mpg). If I think how much I am saving it helps motivate me to drive more efficiently. I am a bit of a reformed accelerator junky - I just didn't realise how much it was costing me. I like the fact it’s better for the environment as well. The app has been really handy to keep me motivated! I found it by searching fuel cost on i tunes. :A

kwaichi

I've been driving efficiently (average 54mpg) in my BMW 320d for a few years now but as a result I developed a clogged up turbo. As I rarely revved over 1krpm I had built up a lot of 'coke' which stopped the air mixing in with my diesel. The repairs cost £2900 from an independent BMW garage.

I now rev up to 4krpm regularly and average 47.8mpg with a clean running engine. I can feel the power under my feet again and have been reminded why I got a BMW in the first place

charlieheard

kwaichi wrote: »

I've been driving efficiently (average 54mpg) in my BMW 320d for a few years now but as a result I developed a clogged up turbo. As I rarely revved over 1krpm I had built up a lot of 'coke' which stopped the air mixing in with my diesel. The repairs cost £2900 from an independent BMW garage.

I now rev up to 4krpm regularly and average 47.8mpg with a clean running engine. I can feel the power under my feet again and have been reminded why I got a BMW in the first place

Diesel is injected straight into the cylinder, which creates the explosion - no sparks, no carburettor and no throttle, just a wide open pipe taking plain air straight into the cylinder via the turbo, so nothing to coke up! Had the injectors themselves clogged up, or was there soot around them?

Depending on the age of the car, you may have a particulate filter on the exhaust. This stores soot particles till the engine and exhaust are hot enough to burn them off. If you do loads of journeys which don't heat up the exhaust enough (i.e. short and/or slow), this will clog up the filter. Was it this that had clogged? In extreme cases, it may cause the engine to burn it's own oil as fuel. The more usual symptom is that it puts diesel fuel into the engine oil, which thins it and causes premature wear. For more info, have a look at Honest John's website - he has lots of examples of the problems these can cause. The answer is to ensure that you regularly get the engine and exhaust hot enough to burn it off.

Another cause may well be driving at such low rpm. A turbodiesel in a car is woefully inefficient at much below 1500 rpm - my Micra wouldn't pull much below 2000. You can feel when the engine in your car starts to respond and then just note when this point is. So when you accelerate, the trick is to make sure that the revs don't drop below this point till you've reached your cruising speed. Also, don't let the engine labour in too high a gear. Many modern cars are better in 3rd or 4th around town rather than trying to get into top at every opportunity.

A warm engine shouldn't coke up, but some cars are now so efficient that they don't warm up for up to 10 miles or more - anything less than that will start to leave soot. So make sure your engine is fully warmed at least once a week, as short journeys harm any engine, be it petrol or diesel.

almillar

Yup, that's the soot that you see puffing out of diesels when the driver actually does put his foot down, that was all clogging up your engine, including turbo. Short journeys are also bad for petrol and diesel cars for the reasons above. Diesel and petrol engines need the occasional blast, right through the rev range in my opinion...

james2005

It seems like they do...


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