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Fuel Saving Tips
- Thread starter Pope33
- Start date
XennoX
Expert Member
TL;DR: Think of driving in terms of energy where your objective is to minimise the amount of work your car has to do to achieve and maintain your desired velocity.
More thorough explanation below. (Wall of text incoming)
What is work?
In physics, a force is said to do work if, when acting on a body, there is a displacement of the point of application in the direction of the force.[sup]1[/sup]
Work is measured in Joules (J).
How does a car do work?
For a car equipped with a (conventional) internal combustion engine (ICE), a fuel-air mixture is ignited in a confined space (the cylinder). This combustion converts chemical energy into kinetic & heat energy. The kinetic energy of the expanding gas forces the piston down in a linear fashion. This linear momentum is converted into angular momentum via the crankshaft which turns the gears in your gearbox, which turn your wheels via some mechanism (constant velocity joint, propulsion shaft, etc).
There are losses throughout this entire system such that the effective energy stored within the air-fuel mixture is diminished quite significantly. The biggest loss is the conversion of chemical energy to heat energy and kinetic energy to heat energy. Heat energy in general cannot be recovered and used to propel the car.[sup]2[/sup]
Why is the car doing work?
To get you moving and to maintain that velocity.
What is the car doing work against?
Where are my biggest energy costs?
This is where things can get hugely technical with equations, graphs, force-body diagrams, etc. So I won't be delving into that. The assumption I'm making here is that you're doing regular highway speeds of between 80 km/h to 120 km/h.
In general the answer is that your biggest energy cost is the energy it takes to get you up to your desired velocity.
Once you have reached a constant velocity, the biggest cost comes from air resistance (drag). Drag is a function of an object's velocity, it's cross sectional area and an empirical coefficient (the drag coefficient) based on it's shape. The higher an object's velocity, for the same cross sectional area and same shape, the more drag the object will experience. Thus your car has to do more work to maintain a velocity of 120 km/h versus a velocity of 80 km/h.
So how should I drive?
Drive analogous to how water flows. Pick the path of least resistance.
That's all I can think of right now.
[sup]1[/sup] Work (physics) - Wikipedia, the free encyclopedia
[sup]2[/sup] If you have a hybrid vehicle that has a kinetic energy regeneration system (KERS), when you brake you convert your kinetic energy into heat energy which then charges the vehicle's batteries. This in turn can be used to propel the vehicle along. This has diminishing returns though.
[sup]3[/sup] Inertia - Wikipedia, the free encyclopedia
More thorough explanation below. (Wall of text incoming)
What is work?
In physics, a force is said to do work if, when acting on a body, there is a displacement of the point of application in the direction of the force.[sup]1[/sup]
Work is measured in Joules (J).
How does a car do work?
For a car equipped with a (conventional) internal combustion engine (ICE), a fuel-air mixture is ignited in a confined space (the cylinder). This combustion converts chemical energy into kinetic & heat energy. The kinetic energy of the expanding gas forces the piston down in a linear fashion. This linear momentum is converted into angular momentum via the crankshaft which turns the gears in your gearbox, which turn your wheels via some mechanism (constant velocity joint, propulsion shaft, etc).
There are losses throughout this entire system such that the effective energy stored within the air-fuel mixture is diminished quite significantly. The biggest loss is the conversion of chemical energy to heat energy and kinetic energy to heat energy. Heat energy in general cannot be recovered and used to propel the car.[sup]2[/sup]
Why is the car doing work?
To get you moving and to maintain that velocity.
What is the car doing work against?
- Inertia (Inertia is the resistance of any physical object to any change in its state of motion)[sup]3[/sup];
- Internal friction (any mechanical component that moves generates friction (conversion of kinetic energy into heat energy);
- External friction (the car needs to overcome the static friction of the tyres against the surface to get moving, and it needs to constantly work to overcome the dynamic friction);
- Air resistance (drag);
- Gravity.
Where are my biggest energy costs?
This is where things can get hugely technical with equations, graphs, force-body diagrams, etc. So I won't be delving into that. The assumption I'm making here is that you're doing regular highway speeds of between 80 km/h to 120 km/h.
In general the answer is that your biggest energy cost is the energy it takes to get you up to your desired velocity.
Once you have reached a constant velocity, the biggest cost comes from air resistance (drag). Drag is a function of an object's velocity, it's cross sectional area and an empirical coefficient (the drag coefficient) based on it's shape. The higher an object's velocity, for the same cross sectional area and same shape, the more drag the object will experience. Thus your car has to do more work to maintain a velocity of 120 km/h versus a velocity of 80 km/h.
So how should I drive?
Drive analogous to how water flows. Pick the path of least resistance.
- Maintain a constant velocity (thus constant momentum) as much as possible. This means, minimise acceleration and deceleration. If you need to slow down, coast/glide to do so.
- Be smooth with your acceleration and your gear changes. You should change gears just before you reach maximum torque. Revving your engine higher to get up to speed quicker costs you due to the increased heat and subsequent drop in efficiency.
- Watch your velocity. Higher velocities cost more energy to maintain.
That's all I can think of right now.
[sup]1[/sup] Work (physics) - Wikipedia, the free encyclopedia
[sup]2[/sup] If you have a hybrid vehicle that has a kinetic energy regeneration system (KERS), when you brake you convert your kinetic energy into heat energy which then charges the vehicle's batteries. This in turn can be used to propel the vehicle along. This has diminishing returns though.
[sup]3[/sup] Inertia - Wikipedia, the free encyclopedia
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one last thing: its not correct to say cruise at 80 or 100 or 110 km/h as that is an assumption that does not hold true for all vehicles.
in top gear, cruise at the speed where you are making the most torque.
for example your car might give peak torque at 3000rpm. therefore in top gear at 3000rpm, your car will be making the most power with the least effort. this means that you do not need to drive at varying throttle opening or even WOT ( wide open throttle ) to be able to maintain the speed you get at 3000rpm.
and if you are not opening that throttle up then you are not allowing more air and therefore more fuel in.
turbo diesels have peak torque coming in early enough, petrol NA cars still a bit more linear as you accelerate with the peak torque somewhat higher up the range. Still- this holds true.
so drive economically: drive where your peak torque is. your car will love you for it, and ur fuel bill will be smiling!
in top gear, cruise at the speed where you are making the most torque.
for example your car might give peak torque at 3000rpm. therefore in top gear at 3000rpm, your car will be making the most power with the least effort. this means that you do not need to drive at varying throttle opening or even WOT ( wide open throttle ) to be able to maintain the speed you get at 3000rpm.
and if you are not opening that throttle up then you are not allowing more air and therefore more fuel in.
turbo diesels have peak torque coming in early enough, petrol NA cars still a bit more linear as you accelerate with the peak torque somewhat higher up the range. Still- this holds true.
so drive economically: drive where your peak torque is. your car will love you for it, and ur fuel bill will be smiling!
supersunbird
Honorary Master
A constant 110km/h if possible on highway.
Polished car.
Tyres a small bit overinflated.
Polished car.
Tyres a small bit overinflated.
True for most diesel engines, but not for petrol.
S2000 is the extreme, but max torque is only at +7000rpm.
All my previous petrol cars (non turbo) only made max torque at +- 4000 rpm, and in top gear it will be over 140kph....
My turbo petrol seemed to be more economical when I accelerated quickly to cruising speed and then holding it, than it did when I accelerated slowly and holding it.
Both scenarios changed gears at approximately the same place in the rev range (3000 rpm). This was in a 60 zone, so I went up to 69km/h max.
I personally would also try not to drive at below 1800 - 2000 rpm in the rev range. Accelerating from lower in the rev range or going up hill from lower in the rev range "feels" to me like it uses more fuel than it does from 2500rpm for example. This is based on pedal travel.
I suggest that if you are serious about this, take two consecutive weeks in Feb, and drive the one accelerating quick to a target speed, and the other as slow as you can. Check fuel consumption.
Both scenarios changed gears at approximately the same place in the rev range (3000 rpm). This was in a 60 zone, so I went up to 69km/h max.
I personally would also try not to drive at below 1800 - 2000 rpm in the rev range. Accelerating from lower in the rev range or going up hill from lower in the rev range "feels" to me like it uses more fuel than it does from 2500rpm for example. This is based on pedal travel.
I suggest that if you are serious about this, take two consecutive weeks in Feb, and drive the one accelerating quick to a target speed, and the other as slow as you can. Check fuel consumption.
diesel wrong.
Bike dangerous.
XennoX mostly right.
A3@MBB Wrong.
supersunbird right about tyres and constant speed. Polish wrong.
pboy completely wrong. F = MA + (Drag coeficient + Rolling resistance) Acceleration = consuming alot of fuel.
Leo wrong.
Bike dangerous.
XennoX mostly right.
A3@MBB Wrong.
supersunbird right about tyres and constant speed. Polish wrong.
pboy completely wrong. F = MA + (Drag coeficient + Rolling resistance) Acceleration = consuming alot of fuel.
Leo wrong.
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And pay out your ass for services, tyres and other consumables. With a bike you save on time in traffic, but a big bike costs about the same to run as a car.
Sinbad
Honorary Master
- Joined
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Lol.
Biggest user of fuel is the brake pedal.
Diesel much more economical round town than petrol.
SauRoNZA
Honorary Master
Yup.
But something like an NC750 should do well compared to most cars. Light on fuel and easy on chains, tyres etc.
We need stop/start in more bikes.
Are you sure? Diesel is no different to a petrol when driven correctly. Also diesel much more expensive to buy and maintain. And it is a messy oil burner...
Sinbad
Honorary Master
- Joined
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- Messages
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Wrong, wrong.
My wife and I had very similar cars. One was a turbo petrol, the other turbo diesel. My petrol was more expensive.
I regularly get under 5l per hundred with hers. I never saw under 7 with mine.
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pinball wizard
Honorary Master
Ya. If you're talking about an ancient diesel. The new ones are just peachy.
Ok Lets start.
One liter of diesel contains the same amount of energy as its petrol couterpart.
in theory fuel consumption should be the same in both types of engines.
Diesel needs turbo charger, Which consumes extra ennergy.
Needs stronger internals which increases inertia and adds alot of weight.
Uses pressure and temprature to combust fuel mixture.
Petrol fueled engines on the other hand are much more advanced. You have a truely versatile engines with a wide band of power delivery and light weight cataristics.
Are we racing Indy 500 series or are we trying to get to work as cheap as possible?
supersunbird
Honorary Master
I would think it is the fuel pedal.
With cars with the those fancy fuel usage computers, hard pull aways the fuel usage shoots up. I tried the heavy braking... no impact really according to that system.
supersunbird
Honorary Master
So air resistance is a myth, got it.
So smooth a magnetic decal slides off it? Yet the air wont flow more smoothly over it at all at speed?