engines, speed and economy....
Discussion
It's not difficult and there most definitely are applicable general rules,
- The faster you go the more work you have to do pushing air out of the way (rises as the square) so the worse your fuel consumption.
- Once the engine is going fast enough to run efficiently and provided the gearing is suitable you will get best mpg at slowest speed.
Worth mentioning that aerodynamic drag force does increase with the square of speed, but as P=Fv, the power you need to overcome that force increases with the cube of speed. That's why 90mph can cause such a hit over 70mph - while you might be operating the engine a little more efficiently, you need 2.1 times the power to do it!
ETA - Bear in mind, however, that as Talksteer points out, this doesn't mean you need 2.1 times the fuel, as despite using it faster, you're travelling faster too - you would in fact need 1.65 times the output from your fuel. Energy requirement per mile, then, varies more or less with the square of speed at motorway speeds.
Edited by McSam on Wednesday 29th February 13:48
Ozzie Osmond said:
It's not difficult and there most definitely are applicable general rules,
in auto mode my car won't go into 5th unless i'm doing at least 50mph IIRC - The faster you go the more work you have to do pushing air out of the way (rises as the square) so the worse your fuel consumption.
- Once the engine is going fast enough to run efficiently and provided the gearing is suitable you will get best mpg at slowest speed.
but agree with what you're saying.
but...why is it more efficient at 2k rpm in 5th, and not 2k rpm in 3rd?
i've noticed the economy in my car between 70 and 80 really doesn't seem to change a great deal, in 5th 85mph works out at 2500rpm - redline is 6500rpm. seems to be very happy with running at 85 and economy isn't too bad, still over 30mpg no problems. does 27mpg at 100mph on cruise
cheddar said:
StottyZr said:
matthias73 said:
Can I ask what you are driving?
I managed well over 55mpg from tamworth to leicester once. All the petrol stations were asleep so I had no choice. The problem with this type of driving, is that it is really, really, really boring.
123d, short gearing so 70+ the economy isn't very good. But as shown with the graph I would have very little wind resistance at 50. I managed well over 55mpg from tamworth to leicester once. All the petrol stations were asleep so I had no choice. The problem with this type of driving, is that it is really, really, really boring.
Also, do the same test in a Audi A3 140 tdi and I'd expect you'd get 85+, its not remotely an accurate guide to the cars fuel consumption.
I found a picture I took of the 80.7 mpg
so I could remember the exact number. Sad, I know 
Devil2575 said:
Nick M said:
My suspicion is that there is a point where the aero drag plateaus out for a bit (maybe because you've done most of the work punching a hole in the air) before ramping up again as you go faster
I wouldn't buy into this.I'd suggest that aero drag simply rises with speed.
McSam said:
t's not the case, actually. People don't think it's "right" to run a petrol engine at low speeds, but as I said above, the maximum power output per unit fuel will be achieved at fairly low revs with around 90% of maximum load. This is actually quite true for both petrol and diesel engines, but below is a graph for a 1.8l I4 non-turbocharged engine:
It's a map of engine brake specific fuel consumption, so the lower the number the better. The y axis is brake mean effective pressure, which is directly proportional to torque output (so gives you an idea of how much load for a given speed), and the x is simply engine speed.
As you can see, this engine is happiest in the 2000-3000rpm range, from maybe 75-90% load. That is the place in which you will generate the most power output per unit fuel consumed - so you want to balance all the other factors to fit in with that. The complications for cruising come when you find that increasing your speed by x% improves engine efficiency by y%, but also increases drag by z% - balancing y and z is the hard bit.
Further to that, this graph has other uses: if you wanted 7 bar BMEP (700 on the scale used), which would equate to almost exactly 100Nm for this engine, there's a clear pattern of the best engine speed to use. You can make that much torque at idle, but you're using 20% less fuel at 1600rpm, and less still at 2500rpm. You can go all the way round to 3200rpm (where 100Nm is about 33.5kW or 45bhp) and still not be using a lot more fuel. This then shows the most efficient way to accelerate with this particular engine - but while the numbers may vary slightly, the trends apply to pretty much any engine.
thumbs up for this It's a map of engine brake specific fuel consumption, so the lower the number the better. The y axis is brake mean effective pressure, which is directly proportional to torque output (so gives you an idea of how much load for a given speed), and the x is simply engine speed.
As you can see, this engine is happiest in the 2000-3000rpm range, from maybe 75-90% load. That is the place in which you will generate the most power output per unit fuel consumed - so you want to balance all the other factors to fit in with that. The complications for cruising come when you find that increasing your speed by x% improves engine efficiency by y%, but also increases drag by z% - balancing y and z is the hard bit.
Further to that, this graph has other uses: if you wanted 7 bar BMEP (700 on the scale used), which would equate to almost exactly 100Nm for this engine, there's a clear pattern of the best engine speed to use. You can make that much torque at idle, but you're using 20% less fuel at 1600rpm, and less still at 2500rpm. You can go all the way round to 3200rpm (where 100Nm is about 33.5kW or 45bhp) and still not be using a lot more fuel. This then shows the most efficient way to accelerate with this particular engine - but while the numbers may vary slightly, the trends apply to pretty much any engine.
McSam said:
Worth mentioning that aerodynamic drag force does increase with the square of speed, but as P=Fv, the power you need to overcome that force increases with the cube of speed. That's why 90mph can cause such a hit over 70mph - while you might be operating the engine a little more efficiently, you need 2.1 times the power to do it!
It may require substantially more power (energy release over time) to go faster but the additional speed means the total time to travel a distance goes down.
Power - proportional to V^3
Fuel economy - proportional to V^2
Talksteer said:
McSam said:
Worth mentioning that aerodynamic drag force does increase with the square of speed, but as P=Fv, the power you need to overcome that force increases with the cube of speed. That's why 90mph can cause such a hit over 70mph - while you might be operating the engine a little more efficiently, you need 2.1 times the power to do it!
It may require substantially more power (energy release over time) to go faster but the additional speed means the total time to travel a distance goes down.
Power - proportional to V^3
Fuel economy - proportional to V^2
As I usually use the cube rule for working out changes in top speed caused by power increases and the like, the fact of covering distance more quickly doesn't usually come into it, so I forgot it! I shall edit my original post, while you do need 2.1 times the power, your fuel consumption will only go up by 65%, so you'd need around 40% more efficient operation to compensate.
Nick M said:
My suspicion is that there is a point where the aero drag plateaus out for a bit (maybe because you've done most of the work punching a hole in the air) before ramping up again as you go faster, and the rpm / throttle opening required to maintain 80mph is an efficient combination as far as the engine is concerned.
Also, McSam - do you work for Ford? That's a Ford iso plot of BMEP against engine speed
Basically, the most efficient speed will the lowest speed you can have a gear that will allow sufficient load that you're at the engines lowest BSFC and decent load.
Edited by GroundEffect on Wednesday 29th February 13:47
GroundEffect said:
Also, McSam - do you work for Ford? That's a Ford iso plot of BMEP against engine speed
Basically, the most efficient speed will the lowest speed you can have a gear that will allow sufficient load that you're at the engines lowest BSFC and decent load.
Ahah, someone recognised it already Basically, the most efficient speed will the lowest speed you can have a gear that will allow sufficient load that you're at the engines lowest BSFC and decent load.
no, it is in fact nicked from a lecturer's material - I'm a second-year automotive engineering student. I thought I'd take the engine details and dates and all off it, but figured it wouldn't be long until someone worked it out - it's even got Ford model codes on the gearing lines..Your summary is, of course, bang on what I took several paragraphs to say!
Edited by McSam on Wednesday 29th February 13:55
GroundEffect said:
No. Very no.
Also, McSam - do you work for Ford? That's a Ford iso plot of BMEP against engine speed
Basically, the most efficient speed will the lowest speed you can have a gear that will allow sufficient load that you're at the engines lowest BSFC and decent load.
So is that essentially is why less powerful cars are more economical? Because the engine operates under higher load more of the time?Also, McSam - do you work for Ford? That's a Ford iso plot of BMEP against engine speed
Basically, the most efficient speed will the lowest speed you can have a gear that will allow sufficient load that you're at the engines lowest BSFC and decent load.
Edited by GroundEffect on Wednesday 29th February 13:47
It worth noting that BSFC is not the be all and end all in overall economy!
Ultimately you car about the total fuel mass consumed over a certain distance travelled (i.e. miles per gallon etc)
So, say you have a choice of cruising at a BSFC of 240g/kW.hr at a load of 10kW, or at 290g/kW.hr at only 5 kW roadload. The total fuel consumed in the second case is LESS. (sure the engine is less efficient, but the overall consumption is less). So whilst you should always drive at the lowest possible BSFC for any given power requirement, you shouldn't increase the mean roadload just to move the operating point to one of higher efficiency.
Unless you "coast and burn" there is no way of avoiding this fact with a conventional powertrain. However with a hybrid that has some form of energy storage system it is possible to do "burst conversion" of fuel energy at an average higher efficiency level for the same mean road load.
For example, say your roadload is 5kW, and you drive for an hr, thats 5kW.hr of energy. Now a conventional sized IC engine is very inefficient at only 5kW output (due to the fact that friction and pumping losses are a large proportion of the output at low load). But with a battery or similar storage system you could only run the engine for say 6 mins rather than 60 (10% of the time). In this case to furnish the same 5kW.hr of energy, you now need to run your engine/generator at 50kW (10x the power. because you're only running it for 10% of the time). Now 50kW is a much better power output for a conventional ICE in order to operate efficently.
However now a further problem arises. Namely, the efficiency of your power storage and the conversion steps necessary to leverage that storage, reduces the overall power transfer efficiency!
A normal mechanical transmission is something like 95% efficient, whereas the total "round trip" efficiency for an electrical system with battery storage can be as low as 50-60%. Hence, unless you can move the mean engine operating point to one which is greater than approx 35% better than the pure "roadload" point, that extra benefit is negated by the poorer transfer efficiency.
Ultimately you car about the total fuel mass consumed over a certain distance travelled (i.e. miles per gallon etc)
So, say you have a choice of cruising at a BSFC of 240g/kW.hr at a load of 10kW, or at 290g/kW.hr at only 5 kW roadload. The total fuel consumed in the second case is LESS. (sure the engine is less efficient, but the overall consumption is less). So whilst you should always drive at the lowest possible BSFC for any given power requirement, you shouldn't increase the mean roadload just to move the operating point to one of higher efficiency.
Unless you "coast and burn" there is no way of avoiding this fact with a conventional powertrain. However with a hybrid that has some form of energy storage system it is possible to do "burst conversion" of fuel energy at an average higher efficiency level for the same mean road load.
For example, say your roadload is 5kW, and you drive for an hr, thats 5kW.hr of energy. Now a conventional sized IC engine is very inefficient at only 5kW output (due to the fact that friction and pumping losses are a large proportion of the output at low load). But with a battery or similar storage system you could only run the engine for say 6 mins rather than 60 (10% of the time). In this case to furnish the same 5kW.hr of energy, you now need to run your engine/generator at 50kW (10x the power. because you're only running it for 10% of the time). Now 50kW is a much better power output for a conventional ICE in order to operate efficently.
However now a further problem arises. Namely, the efficiency of your power storage and the conversion steps necessary to leverage that storage, reduces the overall power transfer efficiency!
A normal mechanical transmission is something like 95% efficient, whereas the total "round trip" efficiency for an electrical system with battery storage can be as low as 50-60%. Hence, unless you can move the mean engine operating point to one which is greater than approx 35% better than the pure "roadload" point, that extra benefit is negated by the poorer transfer efficiency.
StottyZr said:
cheddar said:
StottyZr said:
matthias73 said:
Can I ask what you are driving?
I managed well over 55mpg from tamworth to leicester once. All the petrol stations were asleep so I had no choice. The problem with this type of driving, is that it is really, really, really boring.
123d, short gearing so 70+ the economy isn't very good. But as shown with the graph I would have very little wind resistance at 50. I managed well over 55mpg from tamworth to leicester once. All the petrol stations were asleep so I had no choice. The problem with this type of driving, is that it is really, really, really boring.
Also, do the same test in a Audi A3 140 tdi and I'd expect you'd get 85+, its not remotely an accurate guide to the cars fuel consumption.
I found a picture I took of the 80.7 mpg
so I could remember the exact number. Sad, I know cheddar said:
Back in the real world I average 40.
From a 123d? I'd have thought it would be higher than that. I've been getting a fraction under 50mpg out of my 320d touring, though it's mostly journeys of at least 50 miles minimum. I do cruise at approx the proposed speed limit so not crawling along.OldSkoolRS said:
cheddar said:
Back in the real world I average 40.
From a 123d? I'd have thought it would be higher than that. I've been getting a fraction under 50mpg out of my 320d touring, though it's mostly journeys of at least 50 miles minimum. I do cruise at approx the proposed speed limit so not crawling along.E38Ross said:
OldSkoolRS said:
cheddar said:
Back in the real world I average 40.
From a 123d? I'd have thought it would be higher than that. I've been getting a fraction under 50mpg out of my 320d touring, though it's mostly journeys of at least 50 miles minimum. I do cruise at approx the proposed speed limit so not crawling along.Max_Torque said:
It worth noting that BSFC is not the be all and end all in overall economy!
Ultimately you car about the total fuel mass consumed over a certain distance travelled (i.e. miles per gallon etc)
-snip-
Yes - I touched on this with mentioning the complications for working out an ideal cruise speed. Increase speed by x%, gaining efficiency of y%, but at cost of drag of z% etc. and that's the important bit.Ultimately you car about the total fuel mass consumed over a certain distance travelled (i.e. miles per gallon etc)
-snip-
I was using the BSFC stuff mostly to say where the ideal sort of point for most engines is and why it's there, but the really interesting one is what it implies about the most efficient way to accelerate or deal with hills and things. If you're coming up a steep incline in 5th and at 2000rpm and are having to give it quite a lot of boot, you sort of instinctively think that's inefficient and you should change down to get a lesser throttle position. In fact, that makes it much worse, and being almost wide open in the higher gear at 2000rpm would be better!
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