You’ve probably bought a diesel hatchback or sedan looking at the attractive fuel economy figures of over 20 kmpl and thought you could save quite a bit on the running costs. But guess what? In normal driving conditions, you don’t get anything more than 15 kmpl. Why does this happen?
The thing is ARAI-certified mileage figures are carried out under “standard test conditions”, which include a highway drive, but still even getting these figures is not really possible by most in regular city traffic conditions, although some report better-than-claimed figures on highway drives. Also read: Hypermiling techniques, how to get maximum mileage from your car
Are you looking for petrol fuel economy advice?
If you own a diesel car or have recently bought one, upgrading from a petrol, there are some things you need to know about your car to be able to extract maximum mileage out of it. Here are a few points that should help you, specifically for diesel cars, and of course the standard mileage tips also apply:
All about the right torque
A diesel engine produces more torque at lower rpm (pulling power) and you really don’t need to rev the engine. Look at the specification figures for your car and figure out where it makes peak torque (Eg. 220 Nm @ 2000 rpm). Then keep an eye on the tachometer as you rev up in each gear. Rev to just a little past this rpm limit, e.g up to 2,200 rpm and shift into the next gear, you’ll find the rpm dip to about 1900 and then gradually get back to 2000 as speed increases. Keeping a diesel car in this torque band, makes sure it is making maximum pulling power while sipping minimal fuel.
Don’t over rev the engine
Flooring the accelerator pedal in a diesel car for pickup doesn’t work. You’ll notice that most diesel compact cars make peak power (BHP) at about 4000 rpm and the engine will seldom rev beyond that point. However, torque begins to rapidly trail off after about 2,500 rpm in most diesels, and flooring the pedal only burns more fuel, but does not get you moving any quicker. Therefore, it is better to downshift a gear to keep it in the right rpm range rather than just pressing the pedal to get more power. Also see: Most fuel-efficient small cars in India
Be gentle with the accelerator
With a diesel engine you seldom have to press the accelerator pedal more than half way down. Just pressing it about 20% of the way is more than enough to keep the engine within the power band in each gear. Doing this will ensure it sips less fuel. Many diesels these days are common-rail diesels which don’t have cable operated accelerator pedals, but instead use “drive-by-wire” sensors on the pedal to see how much it is being pressed and the ECU then regulates fuel supply to the engine.
Use engine braking
Diesel engines are high-compression engines and hence have very good “engine-braking” capabilities. Modern diesels also have an “fuel overrun cut-off” which comes into operation when you take your foot off the pedal and let the car slow down in gear. Using engine braking gives you better fuel economy as there is no fuel being used at that point, while also helping you slow down easier, and avoiding wear and tear on your brakes. You don’t have to use the brakes too hard. Anticipate your stops way in advance and slow down using engine braking. Press the clutch only at the last moment – once you press the clutch, then fuel supply resumes to the engine as it gets into “idle rpm” mode. Also read: Real-mileage of the Ford Figo as revealed by the CarToq community
Let the car crawl, don’t ride the clutch
Another strong characteristic of diesel engines is their ability to maintain idle rpm without stalling even when driven in gear without pressing the accelerator. For city traffic conditions in bumper-to-bumper traffic, shift to first gear, release clutch gently and fully, and let the car just crawl forward without pressing the accelerator. Press the clutch fully and get back to neutral when you stop, don’t slip the clutch (use half clutch). Letting the car crawl at idle rpm, also helps save some fuel, although this kind of driving is wasting fuel.
Give it enough time to warm up
Diesel engines function best when they warm up as they depend on the heat generated in the cylinders by compressing air to ignite the fuel. A cold diesel engine burns more fuel till the time it warms up. In cold weather this could take as long as a 10 km to 15 km drive for it to reach optimum running temperature. Therefore if you are not going to be driving that distance, don’t use a diesel car for very short trips. Also see: Most fuel-efficient entry level diesel sedans
Other than these specifics, the standard fuel economy tips apply:
Standard tips for better mileage
- Maintain the correct tyre pressure.
- Reduce weight in the car, by keeping unnecessary junk out of it
- Get the car regularly serviced, keeping air filter clean
- Gently accelerate, gently slow down
- Shift gears at the correct rpm
- Drive at the optimum speed, not exceeding about 60-70 kmph depending on the car
- Switch off the engine if you are going to be idle for more than a minute
- Reduce AC use if possible
Share any other tricks that have helped you achieve good fuel economy with your diesel car in normal traffic conditions – not hypermiling.