ARTICLE: How to get the maximum Fuel Efficiency

[vineethvazhayil]

Originally Posted by petrohead

Does it affect the braking distance ? ( If when an emergency occurs, Indian roads ain't it! )

Yes, on neutral, braking distance increases, and with the unpredictability of traffic on Indian roads, can be extremely dangerous!

[swift8847]

Is it true that in the new cars For eg: Swift the fuel supply to the engine reduces when the accelerator pedal is released?

[71Convertible]

At the risk of bringing back to life a debate which went nowhere:

Fuel injected engines sense many things -- rpm, vehicle speed, manifold vacuum (or pressure in case of turbo) etc.

When you are driving at a certain speed and then let go of the acclerator, the TPS (throttle position sensor) will sense that throttle is closed (it's not 100% - never is) AND manifold vacuum is high. Reason for high vacuum is that the momentum of the car drives the engine to turn though the still engaged transmission. So the cylinders are sucking, but with throttle closed there is not much vol flow so lower pressure. FI system will then cut back the fuel injected. If it did not, the mixture would be too rich as there is not much air going into the cylinders. It may even cut the fuel mixture below the ideal (to almost zero) since it can sense that work is not required out of the engine.

If you stepped on the clutch or shifted to neutral, engine speed will decrease since the car's momentum is no longer being used to turn the engine. With decreasing engine speed, manifold vacuum decreases. Engine senses that and injects fuel according to the idle/no-load portion of the fuel map (whereas while braking it is in the non-idle/no-load portion of the fuel map).

BTW, it is not just the fuel amount the gets change, but also the engine timing I believe.

Now on the question of is it better to idle the car or engine brake the car (purely from an FE perspective) while going downhill or coasting - I don't know. It should be obtainable on cars today that have instantaneous fuel consumption capability on the trip computer. My vectra has it. I will check. But to get it apples to apples, you would have to first engine brake down the stretch and note the speed, and the idle and use brakes to maintain the same speed -- to keep the time and distance elements the same.

[sandeep108]

It seems to be logical that MPFI engines do cut back fuel supply to the engine when the throttle pedal is lifted. This is borne out by the fuel efficiency and power ratios. In the carb days, 40-45 bhp, one usually achieved a maximum of 8-9 km/l. Now with double the power one can achieve even better FE with MPFI.

The fuel mapping algorithms, sensor quality and quantity also play a major role. e.g. in the swift, there is a noticeable lag before engine braking even occurs when you lift off the pedal, especially with the AC on. In the Honda's it is almost immediate.

[appuchan]

Friends, I have managed to get a consistent 15.1 kmpl for my Hyundai i20 Asta now for regular city roundabouts. During my last long trip, I managed 17.2kmpl.

[Roy.S]

Well, I intend to drive with a heavy foot until the fuel prices go up. Better have fun while you can.
My Swift Vxi gives me 11.5 kmpl /16 kmpl (city/highway) in spite of my pushing it.

[gpa]

Originally Posted by Roy.S

Well, I intend to drive with a heavy foot until the fuel prices go up. Better have fun while you can.

I am a very sedate driver, but on ocassions, I do tend to push the car to its limit. With sedate driving in Bangalore city traffic with minial use of the a/c (5%-10%) I was able to derive a FE of 19.57 kmpl.

[Roy.S]

Originally Posted by gpa

....I was able to derive a FE of 19.57 kmpl.

Holy...Thats one hell of a kmpl figure, esp in Blr. Which car returned this? Alto or Santro?

[gpa]

Originally Posted by Roy.S

Holy...Thats one hell of a kmpl figure, esp in Blr. Which car returned this? Alto or Santro?

That would be the Alto I was very careful to drive in a lightfooted manner. Upshifting and downshifting at the right rpm, no unecessary idling and curising where ever possible at a speed of 60 to 65 kmph.

The maximum that I have gotten out of the Santro was 18.45 kmpl.

[fast lane]

DRIVING STYLE

• Avoid clutch riding: Driving with your foot on the clutch results in “wastage” of engine power, which in turn increases your fuel consumption and wears out your clutch rapidly. Do not keep your foot on the clutch, except for when changing gears.

• Minimise Idling: If your car is going to be stationary for an extended period of time, switch the engine off.


I have been using a Maruti Zen Carburated type and a Fiat Siena.

IN the ZEN I have got constantly FE (in City with 60% a/c -evenings in pune are cool so u dont need a/c) of 15 in city and 20 on highways.

In the Siena 1.2 petrol, I get 12 in city with 70% ac and more than 16 on highways with full ac.

My style in city runs is using the THROTTLE pedal only about 20-25% and if required change into lower gear, but maintain throttle at 20-25% only.

Also, if i have to get to the RED SIGNAL , depress the clutch fully and roll to the stop, alos on slopes and areas where I can - depress the clutch fully and roll along.

DOES THIS affect the CLUTCH in any manner - is this practice OK? Need Help on this fact.

[fast lane]

what is the time frame that we need to have for cutting off the engine - is it 1 minute or more? As i understand , restartinmg the engine also consumes some fuel-specially in mpfi engines with fuel pump.

so what should be the time frame to cut off engine - my estimate is about a minute or more - engine should be cut off to get better mileage. below 1 minute of idling is same a srestarting the engine.

any comments guys?
Fast lane.

[supremeBaleno]

fast lane, I think this was answered few pages ago in this thread.

Anyway, for cars of today, any duration upwards of 10-15 secs is OK to switch off.

[swift8847]

Originally Posted by supremeBaleno

Anyway, for cars of today, any duration upwards of 10-15 secs is OK to switch off.

Just a confirmation. You mean if the engine is idle for about 10-15 seconds its advisable to switch it off?

All these days I thought the time period was 2 minutes because I remember seeing this in some advertisement.

[watashi75]

My Swift's mileage has gone up from around 12 to 13+ after I started to use engine braking more often and increased tyre pressure by 2 psi.

I usually drive between 2.5-3K RPM. Being in a lower RPM and higher gear will give you good mileage only on straight and plain roads. If your daily drive involves lots of braking and acceleration or ups and down, being in the RPM sweet spot will give you better mileage.

[arjyamaj]

Originally Posted by swift8847

Just a confirmation. You mean if the engine is idle for about 10-15 seconds its advisable to switch it off?

All these days I thought the time period was 2 minutes because I remember seeing this in some advertisement.

I do recall similar advertisements. However, those were aired/ printed way before the advent of MPFIs, microprocessor assisted engines and fuel systems smart enough to balance your chequebook for you. Somewhere earlier on this thread, I read that today's engines dont really take up that much fuel when you're restarting- when the engine is warm.

So you really dont have to have a maximum idle time of 2 minutes. My guess is that 10-15 secs should be enough to turn off your engine and restart again.