Here are my tips for improving mpg without going slower, not sure about the order

Anticipation, use less breaking and spend less time stopped

Use steady acceleration

Check your tyre pressures

Keep windows closed and aircon off when not necessary

Don't carry around any unnecessary weight (make your passengers walk)

Wash and polish your car, surely clean shiny cars have less wind resistance

Use a bike for short journeys

Try slip streaming, it seems to work for F1 cars........ only joking

It is still interesting to note that after three pages nobodys picked up on the only actual hard fact posted that peak efficiency for a reciprocationg piston engine occurs at peak torque.

So to get peak efficeincy, drive at peak torque, combined with the usual gubbins of coasting down to junctions, nice progrerssive acceleration etc and good forward planning such as that taught by IAM thats the best way of maxing your efficiency.

Given that according to the rolling road data I have for it, my car makes peak torque between 2250 and 3000 rpm and my best mpg figures are achieved when it's kept between 900 and 1500 rpm, it appears this 'fact' may not be entirely factual. Peak efficiency need not occur at peak torque as there's no guarantee that this creates the correct regime for the engine to convert the energy in the fuel into mechanical work (Which is fundamentally what efficiency is). As I understand it the peak power is set by the brake mean effective pressure, which is set by the peak rate at which the engine can burn fuel and air. Torque is power divided by rpm...

Since for almost all situations, an engine will not need to output it's peak power or torque, it will burn less fuel the less load is on it. On diesels in particular this proportionality holds just about till the load is removed completely, in petrols there is a minimum amount of fuel needed for combustion, but thanks to stratified injection this minimum has got a lot lower.

Edited 12 May, 201016 yr by psycholist

Given that according to the rolling road data I have for it, my car makes peak torque between 2250 and 3000 rpm and my best mpg figures are achieved when it's kept between 900 and 1500 rpm, it appears this 'fact' may not be entirely factual. Peak efficiency need not occur at peak torque as there's no guarantee that this creates the correct regime for the engine to convert the energy in the fuel into mechanical work (Which is fundamentally what efficiency is). As I understand it the peak power is set by the brake mean effective pressure, which is set by the peak rate at which the engine can burn fuel and air. Torque is power divided by rpm...

Since for almost all situations, an engine will not need to output it's peak power or torque, it will burn less fuel the less load is on it. On diesels in particular this proportionality holds just about till the load is removed completely, in petrols there is a minimum amount of fuel needed for combustion, but thanks to stratified injection this minimum has got a lot lower.

I should have added the caveat more clearly. Under load peak efficiency is at peak torque by definition. Volumetric Efficiency, read about it then question the 'fact' if you dont believe it. When not under load then use the coast down techniques explained by others above.

Just to prove my point about driving closer to peak torque ( ie Peak VE ) then think of a car that makes peak torque and close to peak torque at low rpm

Oh yes, a diesel, and on the whole they are more fuel efficient, because they spend proportionally more of their operating time close® to peak torque which is as we should know is by mathematical definition peak efficiency.

Volumetric efficiency is the ratio of the swept volume in the engine to the volume of gas actually moved by one stroke of the piston, it's the reason power output tails off as the revs get higher in most engines (Faster revs don't lead to a proportional increase in air intake past a certain speed, less air means less combustion no matter how much fuel is injected) and the reason for the likes of VTEC or other variable valve timing systems, as well as multiple intake/exhaust valves per cylinder in production engines.

The volumetric efficiency can be greater than 1 in turbo engines, but I'm not sure this equates to thermal efficiency as the slower the engine is moving the higher the volumetric efficiency (Not counting tuning the manifolds), so that would suggest that peak efficiency is at the slowest engine speed possible, which might work, but suggests an operating regime much slower than production engines (And pathetic power output, even if the efficiency is high since so little air would be taken through the engine to burn fuel in).

The only reference to peak torque I could find that seems relevant to efficiency is here where the comment is made that for Otto cycles engines (Which petrol and car sized diesel engines can both be approximated as), the peak efficiency is at 70-80% of peak torque because higher torque figures are achieved by overfuelling, meaning less efficient combustion.

Before this turns into a waste of time flame war I want to say that I'm no expert in this area and would like to find out more and would appreciate being directed to better information.

Volumetric efficiency is the ratio of the swept volume in the engine to the volume of gas actually moved by one stroke of the piston, it's the reason power output tails off as the revs get higher in most engines (Faster revs don't lead to a proportional increase in air intake past a certain speed, less air means less combustion no matter how much fuel is injected) and the reason for the likes of VTEC or other variable valve timing systems, as well as multiple intake/exhaust valves per cylinder in production engines.

The volumetric efficiency can be greater than 1 in turbo engines, but I'm not sure this equates to thermal efficiency as the slower the engine is moving the higher the volumetric efficiency (Not counting tuning the manifolds), so that would suggest that peak efficiency is at the slowest engine speed possible, which might work, but suggests an operating regime much slower than production engines (And pathetic power output, even if the efficiency is high since so little air would be taken through the engine to burn fuel in).

The only reference to peak torque I could find that seems relevant to efficiency is here where the comment is made that for Otto cycles engines (Which petrol and car sized diesel engines can both be approximated as), the peak efficiency is at 70-80% of peak torque because higher torque figures are achieved by overfuelling, meaning less efficient combustion.

Before this turns into a waste of time flame war I want to say that I'm no expert in this area and would like to find out more and would appreciate being directed to better information.

I'd say anbody that knows anything about mapping/tuning etc will know a lot about VE and will confirm peak VE is at the same rpm as peak torque on a normal reciprocating piston internal combustion engine :-)

But the real world example of a diesel should be a big hint that what i'm saying is right, as due to their gearing they spend a lot of time closer to peak torque/peak VE and are on the whole more FE in a car

Remember torque at the crank isnt necessarily torque at the wheels ( a gearbox in engineering terms is a torque multiplier ) and VE can only be stated for a stated rpm and power is merely a function of torque ( governed by gearing ) hence on a RR graph with equal torque/power axis the torque and power curves will always cross at 5250 rpm

The mathematical definition of peak efficiency is the highest value of the ratio between the power output and the calorific value of the fuel times the fuel flow rate. I'm not sure how to relate this back to running at peak torque - and I think this gap in my knowledge is the root of this conversation.

The higher maximum efficiency of diesel engines is down to the higher compression ratio they run compared to petrol and the lower fuel consumption is helped by diesel's higher calorific value (Though depending on the grade of petrol, it appear that petrol can be higher) as well as diesel engines not requiring spark ignition, allowing them to burn what's injected instead of needing a minimum amount of fuel to give a stable flame front from the spark plug when the engine is on partial load.

The mathematical definition of peak efficiency is the highest value of the ratio between the power output and the calorific value of the fuel times the fuel flow rate. I'm not sure how to relate this back to running at peak torque - and I think this gap in my knowledge is the root of this conversation.

The higher maximum efficiency of diesel engines is down to the higher compression ratio they run compared to petrol and the lower fuel consumption is helped by diesel's higher calorific value (Though depending on the grade of petrol, it appear that petrol can be higher) as well as diesel engines not requiring spark ignition, allowing them to burn what's injected instead of needing a minimum amount of fuel to give a stable flame front from the spark plug when the engine is on partial load.

The problem here is that i'm not sure you're understanding 'power' . You cant measure Power of a car engine, all you can measure is it's torque and speed. From my above post. Any rolling road operator will tell you what they actually measure is torque and then use some equations and some 'corrections' to give you a best estimated power vaue

You have to think in terms of torque with an engine as all it does is produce a turning force. In fact, think of it as nothing but an air pump and forget about fuel etc as well. Easier airflow = higher VE

Of course you can measure power - you can do it by making the engine do mechanical work like lifting things or accelerating a car, converting the mechanical power output to heat and measuring that, using it to generate electricity etc. Power can be defined as torque (Nm or Joules, these units are the same) times speed of revolution (rad/s) - there are many other equally valid definitions, provided the units you get are Watts = J/s = Nm/s = kgm/s^3, you've got power.

There is no adjustment or fiddle factor needed in this. A rolling road will measure the torque transferred to the rollers and the speed the rollers turn at to calculate power. The car gear ratio is needed to relate this data to the crankshaft rpm and often factors are multiplied in to allow flywheel power and torque figures to be estimated based on what is delivered by the wheels to the rollers (Overestimating drivetrain losses is a great way to claim an engine outputs more power than it really does). Conveniently on most sets of rollers you just tell the computer running the rollers what rpm the engine is doing in your gear of choice and the rolling road can calculate the ratio itself by comparing the roller rpm with the crankshaft rpm it's been given.

Saying you can't measure power directly is like arguing that you can't measure distance, you can only measure the length of time a laser takes to get to the target and back and multiply by the speed of light/2 or the number of times a wheel of known circumference turns while rolling the distance multiplied by the circumference.

In terms of the theoretical modelling of an engine it is one of the primary assumptions that it's just an air pump. I definitely agree that the better the volumetric efficiency is the more easily an engine can breathe and that it will contribute to the overall engine efficiency, but given engines tend to be below 30% thermally efficient while their volumetric efficiencies can be heading for 100% at low rpm, I'm not sure how peak torque can be linked with peak efficiency.

Edited 13 May, 201016 yr by psycholist

Of course you can measure power - you can do it by making the engine do mechanical work like lifting things or accelerating a car, converting the mechanical power output to heat and measuring that, using it to generate electricity etc. Power can be defined as torque (Nm or Joules, these units are the same) times speed of revolution (rad/s) - there are many other equally valid definitions, provided the units you get are Watts = J/s = Nm/s = kgm/s^3, you've got power.

There is no adjustment or fiddle factor needed in this. A rolling road will measure the torque transferred to the rollers and the speed the rollers turn at to calculate power. The car gear ratio is needed to relate this data to the crankshaft rpm and often factors are multiplied in to allow flywheel power and torque figures to be estimated based on what is delivered by the wheels to the rollers (Overestimating drivetrain losses is a great way to claim an engine outputs more power than it really does). Conveniently on most sets of rollers you just tell the computer running the rollers what rpm the engine is doing in your gear of choice and the rolling road can calculate the ratio itself by comparing the roller rpm with the crankshaft rpm it's been given.

Saying you can't measure power directly is like arguing that you can't measure distance, you can only measure the length of time a laser takes to get to the target and back and multiply by the speed of light/2 or the number of times a wheel of known circumference turns while rolling the distance multiplied by the circumference.

In terms of the theoretical modelling of an engine it is one of the primary assumptions that it's just an air pump. I definitely agree that the better the volumetric efficiency is the more easily an engine can breathe and that it will contribute to the overall engine efficiency, but given engines tend to be below 30% thermally efficient while their volumetric efficiencies can be heading for 100% at low rpm, I'm not sure how peak torque can be linked with peak efficiency.

So as you say, you dont/cant measure power @ wheels with a rolling road. Its a calculation on measured torque and some 'corrections'. I agree 100%

Rolling road fiddles........ambient temperature, intake temp correction, air pressure...................tyre pressures, 'measuring' transmission losses etc etc, a RR takes a measured torque figure and makes a calculation with some corrections and 'best guesses'. Ask any decent RR operator, such as a DynoDynamics one who tend to be very well trained then they will happily tell you. But this is another topic

As for efficiency and torque not being linked....then i suggest you read this book or a similar basic one about tuning this extract clearly states peak VE is at peak torque.

Its all about airflow

Edited 13 May, 201016 yr by VRsMatt32

A rolling road will measure power at the wheels. It measures the power output by the car to the rollers on that day, ignoring any other factors. This isn't very useful if you want to compare different cars (Or even the same car) on different days though. To make tests repeatable for the same engine at different atmospheric pressures and temperatures, one set of corrections is needed to bring the figures to a standard temperature and pressure. To get flywheel power, the driveline losses need to be accounted for, requiring another set of correlations. This doesn't mean the RR isn't fundamentally measuring power output though.

Unfortunately the link you posted seems to be missing. Could you post it again please?

A rolling road will measure power at the wheels.

It doesnt, it physically measures torque @ wheels and CALCULATES power.

Link on the way if I can remember the book!

http://books.google.co.uk/books?id=ze_w0i3xZmAC&pg=PA128&lpg=PA128&dq=volumetric+efficiency+%3D+peak+torque&source=bl&ots=mZ0z8cTm9Y&sig=L8D6wy5vfXJBpeogRcgFwM5dgbY&hl=en&ei=3RrtS6OPA9SA_QaDksX-CA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CBgQ6AEwAQ#v=onepage&q=volumetric%20efficiency%20%3D%20peak%20torque&f=false

Thanks for the link. Only the contents page is viewable to me, but thanks to Amazon I've now bought a copy of this book since the reviews are good... Of course Amazon being Amazon I ended up buying 3 other engineering related books that looked interesting and were under a fiver too ...

Thanks for the link. Only the contents page is viewable to me, but thanks to Amazon I've now bought a copy of this book since the reviews are good... Of course Amazon being Amazon I ended up buying 3 other engineering related books that looked interesting and were under a fiver too ...

No Probs, i'm the same, I go on amazon looking for something and end up with loads of others! Its page 128 where it starts talks about VE and Torque and BMEP.

Ace book :-)