Reducing from 30 to 20 MPH increases fuel consumption

[bazhart]

For anyone interested in why reducing speed limits increases fuel consumption and/or how that is connected to our ECO-POWER engine project - please contact [email protected] and ask for the report on the 30 to 20 MPH speed reduction.

Baz

 

[maldren]

and I'm sure it therefore increases pollution, in London they claim pollution kills hence the new ULEZ limits. I wonder which kills more, speed or pollution? I doubt Sadiq Khan would enter the discussion.

 

[benoniboksburg]

I sure the Politicians know what they are doing !

 

[Alex]

Basic physics - lower speed = lower gear = more engine revs per metre = more fuel used per metre.

The mind absolutely boggles on this one.

At some point folk need to wake up and smell the coffee. The whole CAGW (Catastrophic Anthropogenic Global Warming) narrative is false. There's too many climate scientists speaking up against it now to ignore the fact the whole thing is one big political agenda against capitalism/western world.


I watched a good interview with Dr Steve Koonin the other week that expose the BS pushed by politicians. They even contradict what the IPCC claim now:

BCC code doesn't seem to work (may be a site problem, so click on the link here: https://www.youtube.com/watch?v=l90FpjPGLBE

 

[T8]

I've just got back from leading a five car convoy through Wales with
their new blanket 20 mph in built up areas.

5 Porsches doing exactly 20 mph through their empty villages really peed off the locals stuck behind us and felt like a bit of pay back to the dash cam vigilantes that do 40 in a 60 as soon as a nice car appears in their mirrors.

:grin:

 

[bazhart]

You are right Alex (as expected) but there are actually far more issues being combined when you drive slower to 20 MPH that contribute to the increase in fuel consumption.

I have been involved in a lively debate on the SAE (Society of Automotive Engineers) open forum for the last 2 weeks covering all aspects of my claims and am also putting that together in report format which will also be available soon to interested parties.

Among these replies Alex was one making your point which doesn't consider all the applicable changes that will become clearer when you read it.

I also made the point about more deaths worldwide from global warming than caused by accidents.


My answers explain why your assumption about lower gears is right but adds more interesting points and they include why changing the engine specification and running in a higher gear at lower revs instead also help improve economy.


I will not post that here as it might take too long and technical for most people but will send you a copy when it is available and will be happy to debate yours (and other responses from those that read it) on here afterwards.

Baz

 

[Alex]

:thumb:

 

[plavix]

opcorn:

 

[Slayer]

Will a PDK or similar better the situation by any means as they tend to go in a higher gear at lower speeds? :?:

 

[brownspeed]

https://petitions.senedd.wales/petitions/245548
theres another one somewhere to repeal the devolved power to the welsh goverment l

 

[lk993]

Basic physics - lower speed = lower gear = more engine revs per metre = more fuel used per metre.

The mind absolutely boggles on this one.

At some point folk need to wake up and smell the coffee. The whole CAGW (Catastrophic Anthropogenic Global Warming) narrative is false. There's too many climate scientists speaking up against it now to ignore the fact the whole thing is one big political agenda against capitalism/western world.


I watched a good interview with Dr Steve Koonin the other week that expose the BS pushed by politicians. They even contradict what the IPCC claim now:

BCC code doesn't seem to work (may be a site problem, so click on the link here: https://www.youtube.com/watch?v=l90FpjPGLBE

Spot on. Believers I find tend to be simply ignorant of the facts and willing to parrot mainstream programming (Apart from those with some sort of interest in the agenda).

 

[bazhart]

To try and cut a very involved subject down into a simple explanation is very difficult.

The power at the wheel is close to the same in any gear - it is the torque that changes.

But if you use less power from your engine to drive slower then you also have the throttle more shut which means less air is entrapped in the cylinder.

As per Boyle's Law, entrapping less air means it starts out being compressed at a lower pressure and since the C/R is the same - it then reaches a lower compression pressure when it is ignited.

The whole principle behind the OTTO cycles is that compressing the air to a higher pressure (and therefore temperature - universal gas laws) releases more energy from the same amount of fuel than it takes to compress it - leaving you with a net gain to exploit.

The limit is when that compression pressure reaches a temperature that instigates detonation (KNOC) and that will occur when the most air is being entrapped - so both at full (or almost full) throttle and in the rev range where the engine was designed to create the maximum torque.

A graph of torque against revs has the same shape as a graph of BMEP against revs and a graph of specific Torque compares the resulting average combustion pressure on the expansion stroke.

It stands to reason therefore that the higher the torque (or to compare different capacity engines the specific torque or BMEP) has resulted from the higher the compression pressure.

That compression pressure is the product of 3 main influences.

(1) The C/R
(2) The Volumetric Efficiency "VE" (or how much air compared to a full swept cylinder volume @ atmospheric pressure (atm) was entrapped at the start of compression.
(3) Atmospheric pressure (or boost pressure). But using a compressor absorbs too much of the available power at very low power outputs when cruising) so either negates the benefit of higher C/R’s or requires a lower C/R initially which also reduces thermal efficiency when cruising even lower.

The simplistic formula would be C/R * VE * atm but for a naturally aspirated engine since atm is quite constant - the compression pressure reached is a function of the product C/R * VE.

The C/R maxes a small difference but the VE ranges from 0-100% (depending on how open the throttle is and at what revs).

To cruise slowly uses less power (less internal and external resistances) so to cruise slowly you close the throttle from a higher speed.

Less power at the wheels means less torque at the engine and lower BMEP and that means the engine is running at a lower thermal efficiency (using more fuel to create the same power or torque it could have done if the compression pressure was higher).

Back 25-30 years ago industry tried all sorts of mechanical ways to vary the C/R so they could increase the compression pressure at low outputs (or loads) to improve economy (and reduce emissions) and then reduce the C/R again when driven fast to avoid knock - but it seems they all (apart from the clever variable stroke Nissan) proved too expensive.

It seems no one thought of varying the VE (amount of air entrapped before compression) as the alternative to prevent knock.

Despite high claims - the manufacturers generally applied their ideas to smaller capacity engines and then added turbo chargers to match the high end performance of larger engines (which limits the benefits because smaller engines already have to work at a higher proportion of their maximum power output even at slower cruising speeds).

Since there is little difference in the power needed to cruise at the same speeds (if the engine is small or a large capacity powerful one) the bigger and more powerful the engine the worse that thermal efficiency is at slower cruising speeds - so our Porsche are extremely uneconomical at 30 to say 50 MPH cruising.

Raising the C/R makes a huge difference to the reduced amount of fuel you need to cruise because the product of C/R * VE is much higher - but it is still no where near the knock limit - and we have found also improves acceleration, throttle response and of course economy and reduced emissions.

People seem to hate the fact that no one applied this idea before and knock it for reasons that are not true.

Some say NOx is worse because the compression pressure is too high but forget it is only as high at 30 MPH as it might have been at say 32 MPH anyway and hugely below the pressures at full throttle when they become significant. furthermore reducing the "VE" to limit knock means it is no higher when driving fast than it was before anyway (and the car is just as fast there).

There are so many simple ways to reduce VE (to avoid knock) and fitting high C/R pistons is no more expensive - so it puzzles me why the industry did not try it even during the enforced transition to EV's/

Getting back to reduced limits - driving slower means using less power and more emissions as a result.

My own PDK does slip into 6th around 30 MPH cruising and is almost twice as economical cruising @ 70 MPH as @ 30 MPH because it needs higher compression pressures to overcome the increased resistances at higher speeds and therefore burns fuel more efficiently.

A few graphs might bring this point home to more readers.

These show where our engines work when cruising and the graph of thermal efficiency against compression pressure is working at the far left hand end when we are cruising and at the high end when flat out - where you can see that a small increase in C/R (same variable as compression pressure) makes a huge difference at the low end because the graph line is almost vertical whereas it makes little difference at the top end.

it seems everyone was so obsessed with how much power they could create when flat out everyone forgot about the potential benefits of increasing compression pressure where we drive most often and finding as way to limit the resulting knock at the top end (which we use less often) which we found easy to accomplish.


Baz