Do I use less petrol at lower revs?

[deptfordmarmoset]

I don't own a car any more but when I get into the only car I'm insured to drive, I always switch the display to mpg. As it's a small car - Honda Jazz - I'm very aware that if I'm in too low a gear, I have no power available for emergencies when I'm in a ''rolling'' gear. So, there's a trade off between staying with enough revs for the car's limited torque to respond to sudden accelerations and keeping in a gear that responds. I use the brakes only just enough, coasting up to red lights while feathering the brakes only enough to activate the brake lights, that sort of stuff. I'll get it to 60mpg at 60 mph on motorways but only around 40mph in local London roads.

Drive like a lazy cyclist seems to be close to my motto.

 

[Smurfy]

Volkswagen publish some performance and economy information for some of their engines. But unfortunately you have to go to their Industriemotoren website to find it.

This engine (or something quite similar) is probably fitted to various Volkswagen Group vehicles (Volkswagen, Audi, Seat, Skoda), and there's some quite useful info available, but only if you're an industrial designer. I don't know why I couldn't find any information on the Volkswagen Group automotive website, maybe I didn't look hard enough, or maybe they don't provide it because sales people can't explain it to car buyers in a way that sells more cars.

Here's the power in blue (kW) and torque in yellow (Nm) curves with respect to rpm (1/min) for the Turbo Direct Injection (TDI) engine I linked to above

and here's the fuel consumption (g/kWh) with respect to rpm (1/min)

It looks to me like this particular engine is most economic with fuel at 1700-1800 rpm, which coincides with peak torque on the torque and power versus rpm graph. One possible problem is that this engine as fitted in a vehicle may have had the ECU mapped slightly differently to change the shape of the performance and fuel efficiency curves. Anyway, I suppose it's useful as an illustration.

 

[asterix]

You have been watching too much Ferris Bueller.

No, he must have been watching me.

I think we concentrate too much on driving economically. You should drive in a way that is safe..

IMO economical driving is more likely to be safe than uneconomical driving. Nothing that I can understand from your post makes me think otherwise. I have been driving 43 years during which I have had no collisions since a guy ran into the back of me in Ipswich c.1980. I'd been stopped at the lights for a while, he just failed to notice and very kindly arranged for his insurance company to fund a considerable improvement to the appearance of my old banger.

I'm told that driving instructors teach pupils not to slow gradually by changing down through the gears and to use brakes instead. Of course it is much easier to teach someone to pass their test by this approach but maybe that is also the reason why so many drivers don't look ahead very far because they don't have to plan to use the engine speed. Gives them more time to fit a bit of texting in maybe?

Small changes are hard to see, this is known. By slowing down without showing any intent you force the person to pay a lot of attention to the vehicle to see the subtle changes. It ties up concentration time & observation time which would be better served in general environment observations.

Au contraire. If the other person is not paying attention to the vehicle in front then I agree they won't notice it slowing down. Or have I such exceptional eyesight that I quite easily notice such changes in vehicle speed without needing to rely on seeing their brake lights?

 

[swee'pea99]

Volkswagen publish some performance and economy information for some of their engines. But unfortunately you have to go to their Industriemotoren website to find it.

This engine (or something quite similar) is probably fitted to various Volkswagen Group vehicles (Volkswagen, Audi, Seat, Skoda), and there's some quite useful info available, but only if you're an industrial designer. I don't know why I couldn't find any information on the Volkswagen Group automotive website, maybe I didn't look hard enough, or maybe they don't provide it because sales people can't explain it to car buyers in a way that sells more cars.

Here's the power in blue (kW) and torque in yellow (Nm) curves with respect to rpm (1/min) for the Turbo Direct Injection (TDI) engine I linked to above
View attachment 32865


and here's the fuel consumption (g/kWh) with respect to rpm (1/min)
View attachment 32866


It looks to me like this particular engine is most economic with fuel at 1700-1800 rpm, which coincides with peak torque on the torque and power versus rpm graph. One possible problem is that this engine as fitted in a vehicle may have had the ECU mapped slightly differently to change the shape of the performance and fuel efficiency curves. Anyway, I suppose it's useful as an illustration.

There's always one isn't there? Someone who actually does the work and comes up with the answer and spoils a perfectly good ping pong match of ill-informed and passionately held beliefs. Pah! Where's the fun in that?

Interesting though. That curve surely settles it. Consumption is lowest at 'the sweet spot', wherever that happens to be for any given engine, and higher both above and below that spot. So, no point driving at a lower revs in a higher gear on the theory that 'lower revs means fewer explosions and that must use less petrol', because the theory is clearly wrong. I guess engines must use more petrol-per-explosion when the engine is 'working harder', or something. Be interesting to know the 'why' behind that graph. But the 'what' seems clearly unarguable. Thanks Tim. You fun-wrecker you.

 

[paul04]

Also another thing to consider when trying to save fuel, a clean car is better as it slips through the air, a dirty car will cause drag.

 

[tyred]

Also another thing to consider when trying to save fuel, a clean car is better as it slips through the air, a dirty car will cause drag.

That's me buggered then...

 

[snailracer]

[QUOTE 2781086, member: 9609"]Er - you must have missed posts #14 and #46

But remember this is when the engine is at its most efficient whilst delivering power - in a case where you may be travelling at 50 slightly downhill, the engine is barely needed, it is in this situation that i suspect the lowest revs will win the day as spinning the engine at max torque for no good reason is a waste. - probably the best solution in this situation would be to coast in neutral and momentarily engaging the correct gear to gain any lost speed. (some modern trucks are already doing this)[/quote]
Your reasoning is valid for diesel engines, but not petrol.

To idle, a petrol engine burns petrol so it can pull the intake air past the closed throttle plate. This usually uses more petrol than is needed to maintain speed otherwise lost through engine braking (i.e. in gear, not idling) when travelling down a slight downhill. This is why automatic petrol cars don't have freewheels in their transmissions.

With diesel engines, there is no throttle plate constricting the intake air, so idling requires less fuel which means your theory could be true.

 

[Archie_tect]

I recommend taping up the open joints including the bonnet, and fitting thinner tyres.... never carry passengers and leave anything heavy at home including the jack, spare wheel, excess fuel, water in the washers, rear wiper, seats, carpets,.... in fact don't just leave the car at home and cycle.... better still sell it.

 

[snailracer]

...Interesting though. That curve surely settles it. Consumption is lowest at 'the sweet spot', wherever that happens to be for any given engine, and higher both above and below that spot. So, no point driving at a lower revs in a higher gear on the theory that 'lower revs means fewer explosions and that must use less petrol', because the theory is clearly wrong. I guess engines must use more petrol-per-explosion when the engine is 'working harder', or something. Be interesting to know the 'why' behind that graph...

The reason, as best as I can explain, is that the movement of the pistons inside the cylinders cause pressure pulses in the gases inside the engine. The faster the engine spins, the faster these pulses happen. These pressure pulses cause the gases to move back-and-forth inside the engine, which naturally requires energy (i.e. wastes fuel). At a specific "tuned" engine speed, these pressure pulses bouncing around inside the engine cancel each other out, so there is no back-and-forth movement of gases inside the engine, so no extra fuel is needed to overcome these pumping losses.

The tuned engine speed is a function of the size and shape of the inlet and exhaust manifolds and the number of cylinders and their size, and whether a throttle plate is present.

For diesel engines, the tuned speed is about 2000rpm and very peaky. For petrol engines, the tuned speed is about 3000rpm but is much flatter and less distinct.

 

[tyred]

I tired to follow the advice to tape up all openings and make the car more aero efficient so also removed the windscreen wipers. I was driving in torrential rain, couldn't see without the wipers and crashed into a parked van, have overheated the engine due to lack of airflow caused by tape over the radiator grill and now have to pay to fix the van, replace my bumper, bonnet and two headlights, have the head skimmed and for a replacement head gasket. Penny wise and and pound foolish....

 

[Archie_tect]

I tired to follow the advice to tape up all openings and make the car more aero efficient so also removed the windscreen wipers. I was driving in torrential rain, couldn't see without the wipers and crashed into a parked van, have overheated the engine due to lack of airflow caused by tape over the radiator grill and now have to pay to fix the van, replace my bumper, bonnet and two headlights, have the head skimmed and for a replacement head gasket. Penny wise and and pound foolish....

Ah, I see you make the mistake of driving it.

 

[snailracer]

I guess that's the nub of the matter. Any driver knows how to make changes so it 'feels right'. Question is, does that, in fact, minimise fuel consumption (for any given distance travelled)?

As a corollary to that, presumably, if instead of driving so it 'feels right', you gun the engine and use higher revs, intuition says you're probably using more petrol. How about doing the opposite? If you drive in a higher gear, at lower revs, do you also use more petrol? Or do you, in fact, use less?

Car engineers spend a lot of time designing and tweaking the engine and gearbox to balance fuel efficiency, driveability and engine wear to the nth degree, when it is driven such that it just "feels right". If you are driving a car in a manner which feels a bit artificial, then you are probably sacrificing some aspect of the drivetrain.

 

[Smurfy]

There's always one isn't there? Someone who actually does the work and comes up with the answer and spoils a perfectly good ping pong match of ill-informed and passionately held beliefs. Pah! Where's the fun in that?

Interesting though. That curve surely settles it. Consumption is lowest at 'the sweet spot', wherever that happens to be for any given engine, and higher both above and below that spot. So, no point driving at a lower revs in a higher gear on the theory that 'lower revs means fewer explosions and that must use less petrol', because the theory is clearly wrong. I guess engines must use more petrol-per-explosion when the engine is 'working harder', or something. Be interesting to know the 'why' behind that graph. But the 'what' seems clearly unarguable. Thanks Tim. You fun-wrecker you.

Oh dear! My Bad! I should've read the OP

Having started one pointless car-related thread that's spun completely out of whack, I thought I'd take a crack at a second, so...

then I would've realised what was required.

However, in my defence:
1. I've only presented data for one specific engine
2. I've disclaimed slightly by pointing out that the industrial version might be mapped differently from a vehicular installation
3. It was for Turbocharged Direct Injection (Diesel) which still leaves plenty of other engine types to bicker over

Anyway, I'll post a little bit more, before deciding whether to crawl back under my rock as a named and shamed fun-wrecker.

My previous post was for a Turbocharged Direct Injection (Diesel) engine. So I thought it would be useful to post the data for a Suction Direct Injection (non-turbocharged diesel) engine, as there is quite a big difference.

Here's the power in blue (kW) and torque in yellow (Nm) curves with respect to rpm (1/min) for the Suction Direct Injection (SDI) diesel engine.

and here's the fuel consumption (g/kWh) with respect to rpm (1/min)

From these graphs we can see that that the torque curve is actually pretty flat for a non-turbocharged engine, and that a sweet spot for efficiency still exists, albeit at a lower rpm than the TDI engine. There appears to be an efficiency benefit associated with extracting energy from the exhaust gases in the TDI engine, however, many drivers will probably never realise that extra efficiency because the extra power and torque encourages a more agressive style of driving.

If anyone's interested, the web pages are this and this.

 

[Smurfy]

The Audi A3 is the kind of car that the vehicular version of the TDI and SDI Volkswagen engines would go into. As can be seen here, the performance figures can vary quite a bit for the same engine size, some of which might be due to variations in ECU mapping, and some might be due to improved engine design (e.g. gas-flowed cylinder head).

 

[gbb]

You don't need all this opinion, data statistics....just do what I do, drive by the MPG readout on the cars computer. You soon learn where the sweet spots are, what speeds in what conditions are economical and what aren't.

Oh...that'd kill the thread though