ARTICLE: How to get the maximum Fuel Efficiency

[NinadJoshi]

When I was a kid, Pune used to get timely rain and we had three distinct seasons - summer, monsoon and winter - so you may imagine this is from quite a few years ago.

Waddling back home from school in my dreary beige Duckback raincoat on numerous cloudy and drizzling evenings, a dilemma used to invariably pop up in my mind. Would one get more drenched in the rain if he walked slowly through it, or ran fast through it. My eventual answer used to be that it made no difference whether you ran or walked.

By running fast through the rain you subject your body to cut through a larger volume of falling water but the duration spent under the rain shower is lesser than walking through it. Conversely, walking normally through the rain would mean more time under it, but there would be a correspondingly less number of droplets hitting your body since you are moving slower. In the end, the <time under rain * speed of the body> @ constant density of falling water, should produce more or less the same result in either case.

None of my fellow waddling friends ever paid much attention to my theory is another story.

I am faced with a similar conundrum while driving. From a fuel consumption point of view, does accelerating briskly from a standstill make more (fuel) sense than accelerating slowly towards a desired cruising speed ?

BMW apparently researched this one while developing a 5 series some time ago. They concluded that brisk acceleration to the intended speed was more (fuel) efficient than conservative acceleration. In other words, getting into overdrive as quickly as possible by revving relatively high until you get to your cruising speed, has better prospects of saving you a bit of gas than being 'gentle' on the accelerator and reaching your cruising speed slowly. Makes sense because the charge is leaner at higher rpm's than at lower rpms, and pumping inefficiencies are more at lower rpm's than higher - so if I am to go from 0 to my desired steady speed of 80, I might as well do most of it at 3000 rpm and in get there in 8 seconds, rather than do 1800 rpm and take 14 secs.

On the flip side, I am also in agreement with simple physics that says slow or fast, you’ll expend the same amount of power (rate of energy per unit time) to reach your desired cruising speed. If you want to reach 0-80 in a shorter amount of time, you'll use more energy (~ gasolene) and vice versa. So one would - in an ideal world - expend the same amount of fuel in getting to 80 mph in 8 seconds or in 14.

So will one get more drenched by running under the rain or walking under it? Chances are it doesn't matter you'll get equally drenched any which way. Does my childhood theory have an analogy in the vehicle world too?

Mods, I am a newbie and hence ignorant about whether this has already been discussed. If it has, please feel free to scuttle this thread.

[pganapathy]

Very interesting question. I guess if all you do is accelerate hard and get it up to speed and then stay at that speed for the rest of the journey (only possible on highways, but still), then the fuel expended should be the same. Anyway, even if there is a difference, the amount would be minimal.

It is the hard acceleration constantly that hurts the mileage, as is going faster, says speeds of 140 or 160 for example.

Brilliant analogy. Never actually thought of it like that.

[Santoshbhat]

Rapid acceleration to a little higher than the desired speed and then coasting for while is called 'pulse and glide' or 'burn and coast'.
Fuel economy-maximizing behaviors - Wikipedia, the free encyclopedia

As you have mentioned energy losses from the engine is less at higher rpms or on full load. Modern engine ECUs cut off the fuel supply to the engine when the foot is off the accelerator and the engine is running at higher then the idle rpm. The momentum of the wheels basically drives the engine at this point.

[mayankjha1806]

Shouldn't this be part of Fuel efficiency thread?

What i have observed in both my cars is, when driven aggressively i.e. fast acceleration and then maintain desired speed, or slow and steady acceleration to the desired speed, the later leads to better fuel efficiency.

So most of the times i prefer slow and steady acceleration to get to the speed limit. The fast acceleration is reserved for overtaking moves.

[AvinashV]

Is using the ac in blower mode without the ac button pressed the same as using the ac? As in is the reduction in fuel efficiency the same in both cases?

[AutoIndian]

Originally Posted by AvinashV

Is using the ac in blower mode without the ac button pressed the same as using the ac? As in is the reduction in fuel efficiency the same in both cases?

Absolutely not. When the AC is switched on (of course with the blower ON), you will get cooler air, because the AC compressor is working to produce the cooling effect, taking power from engine crank shaft. When the AC is switched OFF & just the blower is blowing (hot/warm) air, you won't get the cooling effect & the AC compressor is not drawing any power from engine crankshaft. Hence theoretically the FE would be more in the second case (with AC off). However for most modern engines there is hardly any (or a marginal) drop in FE with AC ON. Hope this clarifies your doubt.

[raycers_honda]

Which of the following scenario is good for fuel efficiency.And how does it help.
1)Ac on.But driving at speeds not more than 20kph.
2)Ac on.But driving at speeds above 40kph.
In both conditions shifts are done more or less at 2500rpm.

[libranof1987]

Originally Posted by raycers_honda

Which of the following scenario is good for fuel efficiency.And how does it help.
1)Ac on.But driving at speeds not more than 20kph.
2)Ac on.But driving at speeds above 40kph.
In both conditions shifts are done more or less at 2500rpm.

2) should result in better FE.

You cannot have situation 1) without regular use of the clutch (more like stop-go traffic situations) which will affect FE. A/c won't play much of a role here.

[AutoIndian]

Originally Posted by raycers_honda

Which of the following scenario is good for fuel efficiency.And how does it help.
1)Ac on.But driving at speeds not more than 20kph.
2)Ac on.But driving at speeds above 40kph.
In both conditions shifts are done more or less at 2500rpm.

You have not mentioned in either of the scenarios on which gears you are driving. The earlier you upshift (without lugging the engine) the better will be the fuel economy. Assuming you are driving in 2nd gear in case#1 & in 3rd or 4th gear in case#2, the FE will no doubt be better in case#2

[raycers_honda]

Originally Posted by AutoIndian

You have not mentioned in either of the scenarios on which gears you are driving. The earlier you upshift (without lugging the engine) the better will be the fuel economy. Assuming you are driving in 2nd gear in case#1 & in 3rd or 4th gear in case#2, the FE will no doubt be better in case#2

The assumptions are right.Also some times much before approaching a stop light i shift the car in neutral and let it roll upto the stop light.Dont know whether that helps in fuel efficiency in the long run.Not much use of brakes here but only a slight tap to reduce the speed gradually.

[AutoIndian]

Originally Posted by raycers_honda

The assumptions are right.Also some times much before approaching a stop light i shift the car in neutral and let it roll upto the stop light.Dont know whether that helps in fuel efficiency in the long run.Not much use of brakes here but only a slight tap to reduce the speed gradually.

Please do not do this, "shift the car in neutral and let it roll upto the stop light". This makes the car to roll on free wheels, you don't get any assistance of engine braking & stopping the car in case of an emergency would be far more difficult. There is hardly any fuel saving if you coast your car on neutral till the traffic light rather than in higher gear. It is not at all worth taking the risk. When you are nearing the traffic light in higher gear (4th or 5th) just take your foot off the accelerator pedal & let the car coast on the higher gear, due to the inertia it can easily travel upto the signal, w/o you giving any accelerator input

[raycers_honda]

Originally Posted by AutoIndian

Please do not do this, "shift the car in neutral and let it roll upto the stop light". This makes the car to roll on free wheels, you don't get any assistance of engine braking & stopping the car in case of an emergency would be far more difficult. There is hardly any fuel saving if you coast your car on neutral till the traffic light rather than in higher gear. It is not at all worth taking the risk. When you are nearing the traffic light in higher gear (4th or 5th) just take your foot off the accelerator pedal & let the car coast on the higher gear, due to the inertia it can easily travel upto the signal, w/o you giving any accelerator input

Alright mate,thanks for the heads up.

[Chitta Pradhan]

Hi All,

Please have your views on the below mentioned highway drives which should return maximum FE. Car is 1.6 CRDI Fludic Verna with 6 gears. Turbo kicks in at around 1700 rpm.

1. 80 kmph, 6th gear- approx 1500 rpm
2. 100 kmph, 6th gear-approx 1800 rpm
3. 120 kmph, 6th gear- approx 2200 rpm.

I have a feeling situation 1 is most fuel efficient. But fail to understand how can it be at a rpm below the turbo zone. Ideally diesels should deliver maximum FE on top gear round about the beginning of the turbo zone. Thought situation 2 should be most fuel efficient, but probably it is not. May be air resistance at 100 kmph is becoming a hurdle. What rpm do you think should be most fuel efficient for a diesel?

Please respond.

Thanks

[AutoIndian]

You have answered your question yourself only. The increased air resistance at higher speeds is indeed a hurdle to get the maximum efficiency. I have also replied to your query on the Toyota Liva Diesel thread. Just to reiterate over here your car will return max FE at 80 kmph, 6th gear, 1500 rpm which is no where near the peak torque band. Most of the other cars also return max FE @ 80 kmph, which corresponds to 2000 rpm in them. The air drag/ resistance increases exponentially above 80kmph & the engine has to work harder above that speed. Hence @ 110 kmph, when you are running the engine at peak torque, actually it has to do more work to overcome the increased air resistance, resulting in more fuel consumption

[Guna]

Originally Posted by Chitta Pradhan

Hi All,

Please have your views on the below mentioned highway drives which should return maximum FE. Car is 1.6 CRDI Fludic Verna with 6 gears. Turbo kicks in at around 1700 rpm.

1. 80 kmph, 6th gear- approx 1500 rpm
2. 100 kmph, 6th gear-approx 1800 rpm
3. 120 kmph, 6th gear- approx 2200 rpm.

I have a feeling situation 1 is most fuel efficient. But fail to understand how can it be at a rpm below the turbo zone. Ideally diesels should deliver maximum FE on top gear round about the beginning of the turbo zone. Thought situation 2 should be most fuel efficient, but probably it is not. May be air resistance at 100 kmph is becoming a hurdle. What rpm do you think should be most fuel efficient for a diesel?

Please respond.

Thanks

Turbo only helps to achieve higher volumetric efficiency, by force inducing air+fuel and getting more power out of the same engine (capacity). Does it also mean it is most efficient in terms of burning fuel? I think not necessarily. Option 1 must be still most fuel efficient.