[Drivingforfun]

I didn't fully realise how much of a difference it could make

I always assumed I was already relatively economical tending to drive at the limit (on dual carriageways I'll even do ~60 quite often); I don't think I use the brakes nearly as often as most road users. In fact one of the joys of having a Mini is on bends I'll often "lose" someone who was following me closely – not due to acceleration on my part, just having to shave less speed off.

While the roads were quiet on Sunday and yesterday and there was no-one to wind up, I tried driving very economically. My issue seems to be acceleration - both in rapidity & also gear choice... accelerating super slowly (i.e., slower than a lorry would) and changing up early seems to make a bigger difference than I'd assumed.

[Rolebama]

It seems to me that driving within the rpm torque band is more economical than driving in the power band. Something to be aware of though is if you change up too early and the rpm drop too low, then you will waste fuel trying to accelerate, because you can use a little too much throttle and fuel as the engine struggles to maintain or increase the road speed. (Hope that makes sense.)

[olduser]

Originally Posted by Drivingforfun

I didn't fully realise how much of a difference it could make

I always assumed I was already relatively economical tending to drive at the limit (on dual carriageways I'll even do ~60 quite often); I don't think I use the brakes nearly as often as most road users. In fact one of the joys of having a Mini is on bends I'll often "lose" someone who was following me closely – not due to acceleration on my part, just having to shave less speed off.

While the roads were quiet on Sunday and yesterday and there was no-one to wind up, I tried driving very economically. My issue seems to be acceleration - both in rapidity & also gear choice... accelerating super slowly (i.e., slower than a lorry would) and changing up early seems to make a bigger difference than I'd assumed.

Firstly, drag, or air resistance is proportional to the square of the speed (twice the speed = four times the drag = four times the power = fuel x 4).

A car cornering needs more power to maintain the same speed (it takes a force to get it to deviate from its straight ahead path) most drivers will instinctively add a little more gas going around a corner, to maintain speed, the car feels (and is) more stable that way.

Changing speed is expensive in fuel, so high millers do it less often.

High milers, aim to slow, without brakes, to the appropriate speed for a given corner, before the corner, they can do this because they are planing ahead all the time.
High milers try not to use brakes.

Most cars will temporarily richen up the mixture when accelerating, they will also enrich the mixture as throttle is about three quarters open onwards.
In practice, high milers seek to keep the engine and throttle in step with each other. (when accelerating, the gas is increased as the engine gathers RPM rather than a boot full of throttle held until engine catches up )

High milers, get to know how slow they can go in each gear. (gear 2 up change when reached gear 3 minimum, gear 3 up change when reached gear 4 minimum and so on) similarly coming down the gear box.
The aim being to be in the highest gear for as long as possible, this can only be achieved with sensitive use of the throttle.

I suppose the watch words are, smooth ride😎, and no brakes 😮.

Some high milers take their shoes off, claiming a better feel of the throttle but I think they will look a right twit walking across the supermarket carpark in their socks.

Coasting, I would never recommend coasting, there is no specific law against it but the highway code does say the car should be under the drivers control.

[Beelzebub]

Originally Posted by olduser

Firstly, drag, or air resistance is proportional to the square of the speed (twice the speed = four times the drag = four times the power = fuel x 4).

A car cornering needs more power to maintain the same speed (it takes a force to get it to deviate from its straight ahead path) most drivers will instinctively add a little more gas going around a corner, to maintain speed, the car feels (and is) more stable that way.

Changing speed is expensive in fuel, so high millers do it less often.

High milers, aim to slow, without brakes, to the appropriate speed for a given corner, before the corner, they can do this because they are planing ahead all the time.
High milers try not to use brakes.

Most cars will temporarily richen up the mixture when accelerating, they will also enrich the mixture as throttle is about three quarters open onwards.
In practice, high milers seek to keep the engine and throttle in step with each other. (when accelerating, the gas is increased as the engine gathers RPM rather than a boot full of throttle held until engine catches up )

High milers, get to know how slow they can go in each gear. (gear 2 up change when reached gear 3 minimum, gear 3 up change when reached gear 4 minimum and so on) similarly coming down the gear box.
The aim being to be in the highest gear for as long as possible, this can only be achieved with sensitive use of the throttle.

I suppose the watch words are, smooth ride😎, and no brakes 😮.

Some high milers take their shoes off, claiming a better feel of the throttle but I think they will look a right twit walking across the supermarket carpark in their socks.

Coasting, I would never recommend coasting, there is no specific law against it but the highway code does say the car should be under the drivers control.

First, and prehaps a bit pedantically, while air resistance does indeed increase as the square of the speed, the energy needed does not. Not all of the energy is used to overcome air resistance: A proportion is needed to overcome internal friction, to power ancillary functions, and is dissipated as heat or sound.

Second, it seems to be implied that all "high milers" are good and economical drivers, which I'd suggest is far from the case.

[NMNeil]

Automatic transmission and cruise control does all the economy thinking for me. 😎

[Rolebama]

I was taught that the power needed to increase speed by 10mph is roughly double that needed to cruise at a given speed. Eg, if you need 17bhp to cruise at 60mph, then to achieve 70mph you will need 34bhp, even though you may only need 25 to cruise at 70.* This was explained to me at a Technical College when I bemoaned that my 34bhp Austin 7 Mini struggled to reach 65, and without a tail wind and a downhill gradient, 70 was out of the question. The speed of acceleration is relevant to how many bhp you have in hand.
*This basically works for all cars, but more sleek designs require less bhp to cruise at a given speed, so need less to accelerate. Unfortunately my Mini was about as aerodynamic as a brick.

[olduser]

Originally Posted by Beelzebub

First, and prehaps a bit pedantically, while air resistance does indeed increase as the square of the speed, the energy needed does not. Not all of the energy is used to overcome air resistance: A proportion is needed to overcome internal friction, to power ancillary functions, and is dissipated as heat or sound.

Second, it seems to be implied that all "high milers" are good and economical drivers, which I'd suggest is far from the case.

You are right, the increase in friction in an engine is not linear, rolling resistance is more linear but not quite - I was trying to keep it simple.

High Milers - Ought to be better drivers but as you say, many are not, just like drivers in general, some are not.

Perhaps it needs saying, economy should never be at the expense of safety.

[olduser]

Originally Posted by Rolebama

I was taught that the power needed to increase speed by 10mph is roughly double that needed to cruise at a given speed. Eg, if you need 17bhp to cruise at 60mph, then to achieve 70mph you will need 34bhp, even though you may only need 25 to cruise at 70.* This was explained to me at a Technical College when I bemoaned that my 34bhp Austin 7 Mini struggled to reach 65, and without a tail wind and a downhill gradient, 70 was out of the question. The speed of acceleration is relevant to how many bhp you have in hand.
*This basically works for all cars, but more sleek designs require less bhp to cruise at a given speed, so need less to accelerate. Unfortunately my Mini was about as aerodynamic as a brick.

On paper F = m x a where F = Force, m = mass, and a = acceleration.
a = delta v / delta t where delta v = change in velocity, and delta t = time taken in secs.

But added to F there will be a force due to drag, and this will increase as the acceleration proceeds.
When the acceleration is achieved, and we are at our new speed, the force due to acceleration will not be needed anymore but the force due to the new (higher) drag will still be there.

[Rolebama]

It is due to better understanding and use of aerodynamics, that we now need less bhp to hold any given road speed, that we also use less fuel to hold those speeds hence improved economy. Unfortunately, manufacturers now negate some of this benefit by making cars heavier.

[NMNeil]

@Rolebama Also with every new 'feature' that uses electrical power the alternator will take more energy and hence fuel to supply that power.

[Rolebama]

@NMNeil I am in agreement with you 100%. I have an electric water pump and steering - great for the old VOSA consumption test, but obviously with a fully charged battery as the charging system only comes in when needed, but I have to say that cruising at motorway speeds, I do get 40mpg, nowhere near the 57 that the test declared.

[olduser]

Other thoughts - open windows cause a lot of extra drag, such that it is more efficient to close the windows and use the aircon!
Roof racks create more drag, even when empty, the built in roof rails are also draggy.

All cars these days have an exit for air at or near the back, and usually discharge into a low pressure zone.

With rear air exit's, good ventilation can often be achieved with the (heater) open on cold, it may need the fan running to help if there is not enough ram effect. Make sure the pollen filter gets changed when due.

[olduser]

Originally Posted by NMNeil

Automatic transmission and cruise control does all the economy thinking for me. 😎

Savings can be made by gentle use of the throttle this will prevent the kick down coming into play, and enrichment for acceleration.

One taxi I was in recently, I think a KHI, the auto transmission would go from low gear to top gear if that was appropriate.

A cruse control can be too eager to maintain the set speed, wasting a lot of fuel by opening throttle too wide in it's efforts to maintain set speed.

[olduser]

Originally Posted by NMNeil

Automatic transmission and cruise control does all the economy thinking for me. 😎

Savings can be made by gentle use of the throttle this will prevent the kick down coming into play, and enrichment for acceloration.
A cruse control can be too eager to maintain the set speed, wasting a lot of fuel by opening throttle too wide in it's efforts to maintain set speed.

[Santa]

Originally Posted by olduser

A cruse control can be too eager to maintain the set speed, wasting a lot of fuel by opening throttle too wide in it's efforts to maintain set speed.

This is true. On an undulating road, you find the car changing down to keep up the set speed on uphill stretches, where you would normally allow the car to slow down.

[Rolebama]

The first car I drove with cruise control was a customer's Rover Sterling. I had a few junctions on the M40 to get it back. I engaged the cruise control at 70mph. I had to slow quite a bit coming through the M25 interchange and was doing around 40mph when I cleared the junction. (Why do people want to use all 3 lanes to turn off at busy junctions?) So I engaged the cruise control and the cart kicked down and hit nearly peak revs before changing back into top. It kicked down to 2nd gear to apply max acceleration. I then understood why using cruise control on slippery road surfaces is not advised.

[olduser]

Originally Posted by Rolebama

It is due to better understanding and use of aerodynamics, that we now need less bhp to hold any given road speed, that we also use less fuel to hold those speeds hence improved economy. Unfortunately, manufacturers now negate some of this benefit by making cars heavier.

European and UK cars have made a real effort to reduce weight, which is a big contributor towards reducing fuel consumption.

There has been a flirtation with drag reduction but car buyers were not impressed, it takes time for people to accept a car that does not look like their concept of a car shape but we are getting there.

Another factor with drag, is it's not simple, eg. the airflow around the rear of a vehicle looks to be easy to improve until you discover that the low pressure air zone is a big factor in the cars ability to run straight, rather like a sea anchor.

[Drivingforfun]

How would the maths work with an electric car?

From my extremely limited experience driving an electric Mini, my range was affected by speed much more significantly than with liquid fuel - driving at 30mph (not that this would be feasible on a motorway) could triple the range compared with driving at motorway speeds

[olduser]

The maths stays the same but one thing to note, the more efficiently the potential energy (fuel in the tank or kWh in the battery) is converted to work the more closely the energy consumption will follow speed.

In your case, IC Mini v EV Mini and the perceived high consumption of the EV at speed.

How much potential energy do we have?
Useable petrol in tank gives ~ 418 kWh
Useable kWh in EV ~ 38 kWh.

Compare 418/38 = 11 times more potential energy in IC tank but IC can only extract about 20% - 30% of this, the EV efficiency is around 80 - 90 % making the difference nearer 3 times more energy in the IC fuel tank.

What am I getting at?
the capacity gauge in the EV, for the same amount of energy, will fall 3 times faster than the fuel gauge in the IC.
Giving the impression the EV is consuming power faster!

I hope that helps!

[Drivingforfun]

Thank you @olduser ! 👍

In my hamfisted way I did note that at 70mph the EV was working at 75% of its top speed (93mph) almost akin to driving at 6,000rpm (red line at 8,000) whereas the petrol car at 70mph is under half top speed

[Rolebama]

The figures for both an EV and IC car are pretty much the same. It takes form in what power is required to cruise at any speed. For most cars it takes around 23 - 27 bhp to cruise at 70mph. (Roughly 17 - 20kWh). So, just for instance, a 43kWh battery will give a range of 140 to 175miles. Then it is down to the size of the tank to decide the range of an IC, which is usually more than 43kWh equivalent.

[olduser]

Originally Posted by Drivingforfun

Thank you @olduser ! 👍

In my hamfisted way I did note that at 70mph the EV was working at 75% of its top speed (93mph) almost akin to driving at 6,000rpm (red line at 8,000) whereas the petrol car at 70mph is under half top speed

I would guess the EV would be limited to 93MPH by battery capacity/temperature considerations.
But no matter, energy needed will be about the same speed for speed.

I would expect the petrol at steady 70 MPH to running around optimum fuel air, say at 50 MPH it would be running a weak mixture for economy, this also forces the driver to open the throttle reducing the pumping losses which also saves fuel.
Above 70 I would expect it to richen the mixture (an over rich mixture will deliver more power up to a point but not efficient) until at full throttle it would be at it's max richness, what we see is fuel consumption goes up more than expected, with engine on high load. The EMU takes care of all this, following a table stored in the 'chip' (memory).

When a car is modified by re chipping or re coding, the first level is to remove the weak mixture on constant low load, and adjust the ignition timing accordingly, the next level is to start the enrichment earlier, and so on.
When racing of course fuel consumption is not a consideration.

[Drivingforfun]

It's interesting how, like with a lot of stuff, it's not linear nor the same across different cars!

My parents' 1 litre Aygo would do ~75mpg if driving at ~55mph but drops to ~50mpg at motorway speeds

My 2L Mini JCW does 50mpg at those sedate speeds but one time after an emergency I averaged ~85mph and it still did ~38mpg

I guess my observation is the Aygo is relatively frugal when accelerating but expensive to maintain a high speed / my Mini costs more to build up speed & rewards smoothly maintaining a speed with less sensitivity to what that speed actually is

[olduser]

Whilst at speed the drag becomes the major load the power unit has to work against, there are other loads like rolling resistance, transmission losses, and power unit efficiency.

Electric motors can be chosen for high low speed torque, or lower torque at low revs with higher efficiency at higher revs, and there is the gear ratio between motor and wheels, the converter is also adjusting frequency, current, voltage for optimum efficiency and maximum mileage.

The EMU in IC engines, will be adjusting mixture strength, ignition timing, valve timing to make the engine as efficient as possible without exceeding the emission regulations.
Hopefully, the driver is helping with the choice of gear.

The 1 litre engine will be well into its max power output at motorway speeds, hence will be running a richer mixture.

A 2 litre engine will be lower on its power curve at motorway speeds, so is likely to be running optimum fuel air ratio, or even a bit weaker, and will not start running richer mixture until at a higher speed.

All of these add up to a difference between calculations based on drag say should happen and what actually does happen.

[Rolebama]

You have to figure in the fact that in the Aygo running at motorway speeds the engine would probably be running in the power band, whereas your JCS is probably still within the torque band. (My 2L BMW is doing around 2,500rpm at 70mph, well within the torque band.)