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Old 12-03-2015, 08:30 AM   #1 (permalink)
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Help me drive my diesel car more efficiently

I own a Renault Clio IV dci which I am sure none American is familiar with. It is a small diesel hatchback (smaller than a Golf) with a 1.500cc engine (91.5 cubic inches) putting out a whopping 90hp and 220Nm of torque (162 lbft). Claimed FE on the NEDC cycle is 3.6 L/100km (78MPG). Best I have seen so far is 3.7 L/100km (country roads) but a more realistic FC is about 4.2-5 L/100km (67-56MPG). Cruising at the speed limit (130 kmh or 80 mph) the engine is spinning at a fraction higher of 2600 rpm and gets the worse FE of around 5L/100km (56MPG). I don't do a lot of city driving.

My problem is that I don't know what engine rpm is best to shift at when accelerating. I usually accelerate with quite a lot of load in all gears except for first gear (that would spin the wheels believe it or not) and shift at 2000 rpm. But it being a diesel car it is supposed to not have lots of throttling losses so one would assume it is more efficient to rev the engine up till the torque peak (2600 rpm). Also, after I shift, the engine rpm drop to about 1300 to 1500 depending on the gear. I am afraid that I am not driving as efficiently as I could because I am not shifting at the optimal rpm.

What is your opinion? Should I shift at higher or lower rpm? Also, should I accelerate with a lot of load or with minimal load?

I want to post a dyno run but I do not have the required post count of 5.

Gear ratios:
1st: 3.73
2nd: 1.96
3rd: 1.23
4th: 0.9
5th: 0.66
Final drive ratio: 3.55

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Old 12-03-2015, 02:34 PM   #2 (permalink)
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It looks like you are shifting optimally. In theory, you want to accelerate "briskly", meaning not maximum acceleration, but not slowly. Of course while cruising at a constant speed you want to be in the highest gear the engine can handle for that speed.

I never saw particularly good mileage with my diesel Beetle until I started accelerating then coasting. This works well for me as there are lots of hills where I drive.
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Old 12-03-2015, 04:01 PM   #3 (permalink)
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Unless you can find your engine's BSFC, then the rule of thumb is to keep load around 80% and the RPMs in the highest torque range. Then coast...
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Old 12-03-2015, 08:08 PM   #4 (permalink)
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Diesel's usually get their best efficiency at their peak torque. Find that out and you have won half the battle. Sometimes you can find a BSFC map for your engine. That can give insight on how you want to accelerate.

If you have a turbo charger on a diesel install a boost gauge. Driving in a manner that keeps boost low shows you are driving efficiently (DO NOT MODIFY YOUR CAR TO LOWER BOOST).
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Old 12-04-2015, 05:13 AM   #5 (permalink)
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Quote:
Originally Posted by Piwoslaw View Post
Unless you can find your engine's BSFC, then the rule of thumb is to keep load around 80% and the RPMs in the highest torque range. Then coast...
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Originally Posted by ConnClark View Post
Diesel's usually get their best efficiency at their peak torque. Find that out and you have won half the battle. Sometimes you can find a BSFC map for your engine. That can give insight on how you want to accelerate.

If you have a turbo charger on a diesel install a boost gauge. Driving in a manner that keeps boost low shows you are driving efficiently (DO NOT MODIFY YOUR CAR TO LOWER BOOST).
I have tried a lot to find a BSFC map but without any luck. I have found a dyno run though which shows the torque starts coming at about 1800 rpm and rises until it reaches peak torque at 2600. However, I would assume that peak torque comes with high boost. To complicate manners a bit, my car has an eco button that remaps the engine for higher efficiency by reducing throttle response and torque by 10%. Should I drive with the reduced torque map or is it more inefficient?
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Old 12-04-2015, 03:24 PM   #6 (permalink)
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use your eco button and try to keep your rpm around 2600
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Old 12-04-2015, 04:39 PM   #7 (permalink)
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As mentioned above. Keep the rpm in the torque band. I think for you it is better to keep rpm between 1800 and 2600 during acceleration. I think you need to go a little higher in rpm before you shift. You lose the benefit of the spooling turbo if youre rpm drops below 1600rpm.

While cruising. Keep it in highest gear possible without lugging the engine.

Do some aero modifications. Pump up tire pressure to max sidewall or little above.

Drive slower in the highway. 100km/h should give you a bonus of around 0.5l/100km!

Try puls ald glide. But this depends on vehicle to vehicle if it works or not
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Old 12-04-2015, 06:27 PM   #8 (permalink)
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Quote:
Originally Posted by ConnClark View Post
use your eco button and try to keep your rpm around 2600
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Originally Posted by TimV View Post
As mentioned above. Keep the rpm in the torque band. I think for you it is better to keep rpm between 1800 and 2600 during acceleration. I think you need to go a little higher in rpm before you shift. You lose the benefit of the spooling turbo if youre rpm drops below 1600rpm.

While cruising. Keep it in highest gear possible without lugging the engine.

Do some aero modifications. Pump up tire pressure to max sidewall or little above.

Drive slower in the highway. 100km/h should give you a bonus of around 0.5l/100km!

Try puls ald glide. But this depends on vehicle to vehicle if it works or not
Ok thank you very much! I will try accelerating at higher rpm although old habits die hard . I have the tires pumped to max car manual pressure but I don't really want to do any aero mods since it is a brand new car. The good thing is that it has automatic shutters in front of the radiator, a variable displacement oil pump and the cooling system starts circulating the water through the radiator only when the engine gets up to temperature. I do pulse and glide but only in the city or rural roads as I am too lazy to do it on the highway . Actually, 115km/h on the highway give me a 0.5l/100km bonus! (when I am not bored of driving that slow on long trips)
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Old 12-10-2015, 09:39 AM   #9 (permalink)
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I have a similar car, Skoda Fabia 1.4 tdi. At least, similar in that it's a small diesel car. I have found that P+G does not give good fuel economy, nor does accelerating hard (the 80% load mentioned above).
I get best results keeping the rpm low, I rarely go above 2000rpm.
Your car already has one of the best fuel economy mods- the radiator shutters.
Try increasing tyre pressure.
Use any hills to your advantage: I let the speed drop slightly going uphill, and coast down to accelerate again. I do this as aggressively as the traffic allows.
I get 4.3 l/100km average- over some years. Occasionally, in hot weather, I have seen 3.0l/100km!
Good luck!
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Old 12-10-2015, 09:49 AM   #10 (permalink)
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Quote:
Originally Posted by ecoxantia View Post
I have a similar car, Skoda Fabia 1.4 tdi. At least, similar in that it's a small diesel car. I have found that P+G does not give good fuel economy, nor does accelerating hard (the 80% load mentioned above).
I get best results keeping the rpm low, I rarely go above 2000rpm.
Your car already has one of the best fuel economy mods- the radiator shutters.
Try increasing tyre pressure.
Use any hills to your advantage: I let the speed drop slightly going uphill, and coast down to accelerate again. I do this as aggressively as the traffic allows.
I get 4.3 l/100km average- over some years. Occasionally, in hot weather, I have seen 3.0l/100km!
Good luck!
I am going to try both approaches but yours somehow makes more sense to me. I also just ordered a bluetooth OBD thingy to have some more data and try to figure out what's best but it won't arrive till next month or so. My only concern is that this kind of driving may harm the DPF.

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