Ex-Aston Martin CEO claims 20mph speed limits mean drivers are "swarmed by bicycles overtaking from all angles raising the risk of collision"; Wholesome content as Luke Rowe's son does team car TT radio messages + more on the live blog

Awavey wrote:

Indeed and I was always taught to drive in the higher gear to use the engine that way and control speed more easily, this ex Ceo sounds like he shoves it in 1st and revs the engine high to do 20mph which, purely on engine speed means its using more fuel, creating more emissions, than if he kept the revs lower in a higher gear.

On the other hand, I was taught to drive in a lower gear, because then it feels faster and you are less likely to speed.

That depends on whether the engine revs are in a useful part of the torque curve. If you are below the bottom of the torque curve, you will use less fuel in a lower gear/ higher revs.

Back in the day (70s?) car fuel consumption only used to quote constant speed figures, and there was an obvious correlation between increased speed and increased fuel consumption. Then it was changed to simulated urban driving which included more accelerating, idling etc. In some peopls minds this has created the impression that cars are most efficient at 56mph (90kph) because that is now the only constant speed figure quoted.

For what it is worth out 'ere in the sticks I frequently drive at a steady 20mph because I am following a traaactor and I can assure you the instaneous fuel consumption my car shows is very low, and far better than at higher speeds.

This is true for older engines with gearboxes, I don't think it applies to modern EVs.

Pretty much correct. It was often said cars are most efficient at around 45-55 mph cruising speed since you're probably in an overdrive gear. Thats why car makers would quote fuel economy as urban / combined etc. You usually slow down and accelerate more in urban areas due to junctions etc which eats up more fuel. Electric cars still suffer from this a little but since they are much more efficient at accelerating this is less of an issue. 

Car Delenda Est wrote:

Driving, in a constant straight line, going 20ish will create more emissions than going 30-40ish as I understand it. Not sure if that's just because that's the speed engines are deliberately optimised for though, and I don't think it applies to Palmer's precious EVs. However in the real world accelerating to the next red light then braking causes far more emissions.

Come on - you ride a bike, you know that this isn't true.

Aerodynamic drag increases with speed squared - so as you double speed, you have to overcome 4x the aerodynamic resistance.  Driving at 40mph requires the production of twice as much force as driving at 20mph, which common sense dictates must require more energy - whilst there might be some delta in how the energy requirement matches up, an some engine tuning might implement this, I would be amazed if it was sufficient to overcome the increased energy cost of overcoming 4x the draft.

Very true I'm assuming this is a 'fact' that comes from a test bed engine under simulated load rather than from the real world.

Carior wrote:

Car Delenda Est wrote:

Driving, in a constant straight line, going 20ish will create more emissions than going 30-40ish as I understand it. Not sure if that's just because that's the speed engines are deliberately optimised for though, and I don't think it applies to Palmer's precious EVs. However in the real world accelerating to the next red light then braking causes far more emissions.

Come on - you ride a bike, you know that this isn't true.

Aerodynamic drag increases with speed squared - so as you double speed, you have to overcome 4x the aerodynamic resistance.  Driving at 40mph requires the production of twice as much force as driving at 20mph, which common sense dictates must require more energy - whilst there might be some delta in how the energy requirement matches up, an some engine tuning might implement this, I would be amazed if it was sufficient to overcome the increased energy cost of overcoming 4x the draft.

Prepare to be amazed!

The difference between bikes and cars (especially ICE cars) is that bikes are very efficient - with upwards of 90% of energy exerted at the pedals being used to drive the wheels [1][2][3]. Therefore, there is a very direct relationship between the power you need to exert and the external factors such as wind resistance and gradient. 

With a ICE car, the "tank to wheel" efficiency may be as low as 10-20%, with the vast majority of the energy being wasted internally [1][2]. Therefore, whilst going faster will need more power at the wheels, the total increase in energy use will not correspond to the same extent. A small proporiton of that is drivetrain losses which will correlate with speed, but a large proportion is lost in the engine itself and will not. Therefore, an increase in speed (requiring greater power expenditure at the wheels) will not correlate so closely with total energy consumed by the engine.

This is backed up by numerous studies that demonstrate the most efficient speed (assuming constant speed) is typically ~30 - 50mph - see e.g. [1][2][3][4].

There are real world nuances to such a claim that makes ICE vehicles fall very much into either camps - dependent on engine RPM and best suited gear ratio available (especially manual transmissions which have at most 6 speeds). If you have a gear ratio that runs in a gear at 20mph that sits the engine at its most optimum torque delivery (between 1400 and 1800rpm dependent on fuel type, size and engine layout, where an engine in isolation is its most efficient) sitting at 20mph will be more efficient. Optiomum fuel efficiency in combustion engines requires a balancing of a combination of drag (both CdA and rolling resistance), drivetrain gear ratios and engine speeds. Vehicles designed better for "motorway miles" have gear ratios that give peak torque delivery at lower fuel consumption in the 50-60mph range, any higher will succumb to the speed x drag squared problem. For city cars and smaller vehicles, the higher gear options are optimised for 20, 30 and 40 mph respectively and are much less efficient going faster than that.

Multiple stoppages and accellerations which happens more often in inner towns and cities drastically increase fuel consuption and accellerating to higher speeds, regardless of drivetrain takes more fuel consumption to create the power to build up the inertia.

That used to be the case. Constant speed gives pretty low NOx and PM emissions in modern engines. Most tailpipe emissions come from short spikes during acceleration events. The data is well hidden behind emissions data, though, which are almost all presented as average speeds.

Carior wrote:

what's this guy got a doctorate in? and where from because the standards of that programme are clearly very low!

Engineering Management. I assume that's a flavour of doctorate in paper pushing, like an MBA is for Business.

Bigfoz wrote:

Carior wrote:

what's this guy got a doctorate in? and where from because the standards of that programme are clearly very low!

Engineering Management. I assume that's a flavour of doctorate in paper pushing, like an MBA is for Business.

Like people who study "History of Art" because they aren't artistic.

brooksby wrote:

Like people who study "History of Art" because they aren't artistic.

As the graffito above the toilet roll said "Humanities degrees: please take one"