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Old 8th June 2011, 09:52   #1
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Default Energy loss in a vehicle

I tried to find it on the internet without much success, so here's a question:

what factors influence the inefficiencies and losses and how much are those losses at:

40kmph in 3rd gear or 4th gear
80kmph in 4th or 5th gear
100 kmph in 5th gear

Some of the usual suspects (I guess) would be:

engine thermal loss - higher at low speeds, lower at high speeds (as a fraction of total losses)
wind resistance - higher at high speeds
transmission losses - dependent on torque and rpm (can someone throw some light on this?)
tyre rolling resistance - I have no clue how much it is and how it changes with speed
....


Let's start a discussion

EDIT: let's include other vehicles (trucks, buses ...) also

Last edited by vina : 8th June 2011 at 09:56.
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Old 8th June 2011, 16:29   #2
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Default Re: Energy loss in a vehicle

Quote:
Originally Posted by vina View Post
what factors influence the inefficiencies and losses and how much are those losses at:

40kmph in 3rd gear or 4th gear
80kmph in 4th or 5th gear
100 kmph in 5th gear

Some of the usual suspects (I guess) would be:

engine thermal loss - higher at low speeds, lower at high speeds (as a fraction of total losses)
wind resistance - higher at high speeds
transmission losses - dependent on torque and rpm (can someone throw some light on this?)
tyre rolling resistance - I have no clue how much it is and how it changes with speed
....
IMO, the major loses that count are Transmission loses and Wind resistance (Drag).

Transmission loses when compared to the overall loses can be assumed as a constant. And can be obtained by measuring W(wheel)HP. WHP - C(Crank)HP or rated power/torque = Transmission loses. Its usually 15-25%. Worst case being 30%.

Wind resistance or Drag: Is shape is a constant, the drag is proportional to the square of the speed. ie, drag at 100km/h is four times the drag at 50km/h. Again, for making shape favorable, close the windows to allow better air flow.

Well, regarding the other two loses, I guess when compared to the total loses, they are negligible.

Another loss that manufactures account for these days is the loss of momentum while braking. This loss is minimized by converting and storing the energy in electric(?) form.

Quote:
engine thermal loss - higher at low speeds, lower at high speeds (as a fraction of total losses)
I'm sorry but I do not get what you mean. What do you mean by engine thermal loss and how is it higher at low speeds and vice versa?.
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Old 8th June 2011, 16:36   #3
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Default Re: Energy loss in a vehicle

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Originally Posted by dhanushs View Post
I'm sorry but I do not get what you mean. What do you mean by engine thermal loss and how is it higher at low speeds and vice versa?.
the engine heats up due to the fuel combustion and friction. this heat is a loss, since it is not going to be utilised in any way.

You could argue that all engine have an optimal working temperature and this heat helps build that up. but what about the heat that is removed by the coolant ? it cant be utilised in any way other than for heating the cabin in winters. ofcourse unless you talk of Scandinavian environment.
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Old 8th June 2011, 16:50   #4
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Default Re: Energy loss in a vehicle

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the engine heats up due to the fuel combustion and friction. this heat is a loss, since it is not going to be utilised in any way.

You could argue that all engine have an optimal working temperature and this heat helps build that up. but what about the heat that is removed by the coolant ? it cant be utilised in any way other than for heating the cabin in winters. ofcourse unless you talk of Scandinavian environment.
Ah..Now I get it. What got me confused is the "higher at low speeds, lower at high speeds"

So, if the heat produced while combustion is also factored as loses, then higher rpm's would mean more heat generated and hence more inefficient. I dont think speed has something to do with hear generated. Or is it like... as more heat is taken away from the engine, and heat loses are minimized? Well, temperature is lowered, but isn't loss higher?Thanks, for the clarification BuRnT RuBbEr!
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Old 8th June 2011, 16:56   #5
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Default Re: Energy loss in a vehicle

heat 'generation' in the engine should be treated as a function of engine rpm, one would say. the higher the rpm, the more the number of combustion cycles per unit time. since heat is not the desired form of energy in this case (it is, but only to the extent of causing quick expansion of gases in the bore) , it shall be treated as loss.

one of the major 'losses' in an engine would be the energy required to keep the engine turning. All the effort required to open valves etc. is essentially not resulting in WHP.
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Old 8th June 2011, 17:35   #6
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Default Re: Energy loss in a vehicle

Heat loss from engine is a function of RPM, coolant temperature and the ambient temperature.

During the cold start (when nothing is hot) - engine is not efficient and hence the usgae of aeroshutter (Mercedes-Benz & BMW) is quite important - even when thermostat is open, air flow through radiator is restricted. Also, transmission losses are quite significant during this time as the oil temperature is not optimal.

At speeds 60 kmph and above wind drag is significant and the losses are proportional to the speed*speed*speed.
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Old 9th June 2011, 07:18   #7
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Exclamation Re: Energy loss in a vehicle

Vina,

An internal combustion engine (petrol or diesel) uses only about 30% of the chemical power in the fuel to actually move. The rest is all loss in various forms, mainly frictional & heat.

Friction of an ICE ( that is internal combustion engine, NOT in car entertainment) has the main component in the cylinder linings. The higher the RPM, the more the friction, also, the more the number of cylinders, more the friction.

Again the above is somewhat balanced by the fact that, at higher rpm, the heat of combustion stays in the cylinders & does more work, rather than seep through the cylinder walls & heat up the oil & water.

Wind resistance increases as the square of the speed. Yup, so at 100 kmph there is 4 times more wind resistance than at 50 kmph! In fact the highest speed a car manages to achieve is due to the fact that MOST of it's power is going in pushing air out of the way!

Transmission losses is around 5-10%. This is a fact. There is a Norwegian web-site i visited last week (will dig it up) which has tested over 400 cars on the dyno very scientifically & they have the above claims.

Tyre rolling resistance is definately a part of the overall resistance. Can't fix a definate number on it. I expect it to be in the region of 1-2%.
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Old 9th June 2011, 07:54   #8
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Default Re: Energy loss in a vehicle

Quote:
Originally Posted by BuRnT RuBbEr View Post
heat 'generation' in the engine should be treated as a function of engine rpm, one would say. ....
Correction: BHP developed is the yardstick, not RPM.
You can have max rated RPM at idling though very little fuel would be consumed.
Conversely, you can have mid-range RPM but very high load/BHP.
Heat generation is proportional to the amount of fuel burnt.
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Old 9th June 2011, 09:43   #9
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Default Re: Energy loss in a vehicle

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Originally Posted by anupmathur View Post
Correction: BHP developed is the yardstick, not RPM.
You can have max rated RPM at idling though very little fuel would be consumed.
Conversely, you can have mid-range RPM but very high load/BHP.
Heat generation is proportional to the amount of fuel burnt.
How do you achieve that rpm without energy? While i agree that fuel consumption would be more in load state, we can not say that fuel consumption at 6k rpm(neutral)~=fuel consumption at 1k rpm(neutral). Also, if this was the case, you would not witness better mileage for the same speed at higher gears ( ofcourse non-lugging scenario).

heat generation is ofcourse a function of fuel burnt. But here, we are trying to gauge the efficiency of the engine for its output, which is torque and rpm.
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Old 9th June 2011, 14:49   #10
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Default Re: Energy loss in a vehicle

An engine at idling will consume approximately 15% (max of 20%) of it's rated fuel consumption (at full load) in the RPM range from zero to max RPM.
It is the BHP developed that makes all the difference to the fuel consumption.

If you were to keep an engine stationary at rated/max RPM (neutral gear) it would consume about 15% of it's max fuel consumption. Increase the load at this same RPM and you'll find the fuel consumption rising rapidly to the maximum rated consumption, ie from 15% to 100%.

Which is why the specific fuel consumption for engines is stated in grams of fuel consumed per BHP per hour, not grams per RPM per hour.
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Old 9th June 2011, 20:49   #11
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Default Re: Energy loss in a vehicle

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Originally Posted by 1self View Post
Again the above is somewhat balanced by the fact that, at higher rpm, the heat of combustion stays in the cylinders & does more work, rather than seep through the cylinder walls & heat up the oil & water.

Wind resistance increases as the square of the speed. Yup, so at 100 kmph there is 4 times more wind resistance than at 50 kmph! In fact the highest speed a car manages to achieve is due to the fact that MOST of it's power is going in pushing air out of the way!

Transmission losses is around 5-10%. This is a fact. There is a Norwegian web-site i visited last week (will dig it up) which has tested over 400 cars on the dyno very scientifically & they have the above claims.

Tyre rolling resistance is definately a part of the overall resistance. Can't fix a definate number on it. I expect it to be in the region of 1-2%.
Hi,
Let me add to this.

Wind resistance - A polynomial best describes it. At city speeds ~ V. Highway speeds ~ V**2. And if you are emulating a low flying aeroplane ~V**3.

The much bandied about 25% transmission loss is I think a conspiracy hatched by tuners and modifiers. Even the 10% I'll put on the higher side. Anyway, I've stopped arguing about this a long time back.

Tyre rolling resistance is quite significant.

I don't think Vina was asking about the efficiencies of the IC engine, but of what happens to the power produced by the engine.

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Old 9th June 2011, 21:35   #12
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Default Re: Energy loss in a vehicle

Quote:
Originally Posted by vina View Post
what factors influence the inefficiencies and losses and how much are those losses at:

40kmph in 3rd gear or 4th gear
80kmph in 4th or 5th gear
100 kmph in 5th gear
I will perhaps be a listener on this topic. But I felt that the parameters you have written needed an additional clarification. I guess you mean the vehicle is running on a level ground, not on an incline or decline.

But here are my thoughts (anyways)

Rolling resistance = mostly constant with slow increase
Transmission loss = constant
Engine loss = constant as coolant takes care of excess energy in form of heat transfer and that energy is passed on to atmosphere by radiator.
Wind resistance - critical factor at higher speeds.

So at constant speed, no acceleration

At 40kmph 4th gear is more efficient than 3rd as gear ratio is lower.

80 kmph at 5th gear is more efficient

and 80kmph>100kmph at 5th gear because of higher RPM and wind resistance.

Willing to accept criticism.
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Old 9th June 2011, 22:49   #13
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Default Re: Energy loss in a vehicle

Hi, Thanks for the insight.

Quote:
Originally Posted by Sutripta View Post
Wind resistance - A polynomial best describes it. At city speeds ~ V. Highway speeds ~ V**2. And if you are emulating a low flying aeroplane ~V**3.
Is it?. I always thought it to be **2.
Quote:
Originally Posted by Sutripta View Post
The much bandied about 25% transmission loss is I think a conspiracy hatched by tuners and modifiers. Even the 10% I'll put on the higher side. Anyway, I've stopped arguing about this a long time back.
Again, thanks for the insight.
Quote:
Originally Posted by Sutripta View Post
Tyre rolling resistance is quite significant.
Can you please give me an idea about the proportionate loss?. ie, when compared the total power produced?. I thought it was negligible.
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Old 10th June 2011, 18:04   #14
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Default Re: Energy loss in a vehicle

Quote:
Originally Posted by dhanushs View Post
Ah..Now I get it. What got me confused is the "higher at low speeds, lower at high speeds"

So, if the heat produced while combustion is also factored as loses, then higher rpm's would mean more heat generated and hence more inefficient. I dont think speed has something to do with hear generated. Or is it like... as more heat is taken away from the engine, and heat loses are minimized? Well, temperature is lowered, but isn't loss higher?Thanks, for the clarification BuRnT RuBbEr!

Heat generation will be a function of rpm and also to some extent (for ECU controlled cars) power required from the engine.

Normally at higher speeds you'll be cruising in a higher gear, and lower speed in a lower gear, so if power output of the engine is ignored for a given rpm, overall loss will be more at lower speeds. that's what I meant. Power output can not be ignored in reality, but even with that included the overall heat generation (per km for example) will be more at lower speeds.
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Old 10th June 2011, 20:27   #15
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Default Re: Energy loss in a vehicle

Quote:
Originally Posted by dhanushs View Post
Is it?. I always thought it to be **2.

Can you please give me an idea about the proportionate loss?. ie, when compared the total power produced?. I thought it was negligible.
For estimating max speed - power reqd (or rather changes in these two) the predominant term is **2. Which is why everyone is familiar with it.

Tyres - ~ 1% of the weight of the vehicle. Doesn't seem like much, but at low speeds, it's a significant dissipative force.

No mention anywhere of the energy carried away by the exhaust. Or of pumping losses.

Regards
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