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groar 11-01-2008 10:11 AM

How do you eco-drive a diesel ?
Lately a lot of papers are talking about lower consumption and lower emissions (at least about CO2) of diesel. In eco-driving forums it seams that more and more people would like to have a diesel.

As half French people, I have diesel cars. If I have rather good percentage over EPA for the megane, I have difficulties with the scenic (see my signature). Of course the facts the scenic is heavier (2844 lbs vs 2491), has a bigger Cd (0.33 vs 0.31) and a bigger Frontal Area (25.3 sq.ft. vs 21.4) doesn't help, but Combined EPA ratings is better for the scenic (39.8 mpg(US) vs 35.1).

On both cars I added a front grill block and it helps to have the engine hot quicker :) I'm also using DFCO and/or NICE-On coasting, keeping a lower steady speed and having moderate accelerations. I have iMPG display only in the scenic. I can have 2.3 l/100 at a steady 70 km/h (100 MPG at 43 MPH), but it's difficult to achieve and any move of the right feet or change in road inclination will dramatically make the consumption greater and/or the speed lower.

I'm wondering if there is some advices more toward to diesel than gasoline. After a quick search I didn't found any advice on how to eco-drive a diesel except to shift up at 2000 rpm instead of 2500 for a gasoline, but I'm already doing it...

What are the eco-driving and eco-modding tips that are working for your diesel ?

Thanks in advance,


vtec-e 11-01-2008 12:37 PM

Hi Denis. Good thread. As more people drive diesel cars, this is going to become a big issue. My other car is a kia cee'd sw 1.6crdi and i too find it difficult to get it's consumption down. Only on long drives does it warm up enough to become efficient. Anything up to an hours drive just doesn't work. All it seems to do is 5L/100k no matter how i drive it. But once the engine and gearbox is hot, it will do around 2.5 to 3L/100k at 80 to 100kph on the flat. Not too shabby at all! I get up to speed slowly and keep the rpm just shy of 2000. But the warm up hit is so bad that my trip average will be that magical number: 5L/100k!!
I have the rad 60% blocked up and used to have the tires at 50psi but wore more in the middle so they are at 38psi now. Feels like a pig! New tires next year might solve that.


COcyclist 11-01-2008 01:40 PM

Driving Diesels for Economy

Here is a link to that has pages of info about driving the VW TDI for economy. I drive an 04 TDI golf manual transmission and regularly get 50+ mpg on the highway. It is a long thread and I don't agree with all of the opinions in there but it's a good place to start.

Driving for better mpg - TDIClub Forums

I only use engine-on coasting (EON-C?? vs EOC) due to my concerns about turbo long-term durability and the fact that our diesel engines are much more efficient than gassers at idle, when warm (and we lose the use of our airbags when the key is off). My Scanguage shows instantaneous mpg figures in the 300-450 mpg range when EON-C. Also there are times, like on long grades or approaching a red light when it is more efficient to downshift than to coast. Under engine-braking conditions the computer completely shuts off the fuel supply to the engine. Scanguage figures go to 9999 mpg showing no fuel being used while coasting in gear (unless you start lugging it). The downside is that engine friction will slow the car down more than coasting in neutral so you don't go as far before you have to use the engine to get the car up to speed. Of course aerodynamic improvements will increase your coast times and help mpg at highway speed.

Hope this helps. Welcome, it's always good to see another diesel driver here.:cool:

Big Dave 11-01-2008 02:45 PM

Engine-on coasting is my favorite tactic. It even works in urban areas. Get it moving fast enough to get to the next stop and put it in neutral.

Most diesels have some sort of warm-up feature that allowsthem to warm up enough to have an effective heater. Mine came on in September and worked til May. The EBPV on a Ford has a distinctive whooshing sound, so you know its working. Mine worked way too much. So I electrically disabled it. Thay gained me 2 MPG back in the winter. Downside: It takes about 6 miles to warm up enough to make the heater effective.

COcyclist 11-01-2008 06:16 PM

Diesel Ecodriving
Our engines don't have to fight against the vacuum from the throttle plate a gasser has so they are more efficient, but also don't warm up as fast. I have noticed my car gets much better mpg when everything is fully warmed up. My VW diesel has 3 extra glow plugs that heat the coolant when the engine is cold. The coolant heater glow plugs turn off at approximately 180 degrees. I have left them operational for quicker warm-ups but added a plug-in coolant heater to help the engine get to temperature quicker and the glow-plugs to shut off and reduce the load on the alternator. Metrompg did a test of these engine heaters in his car. These could help diesel mileage when the engine has to cold start. I try to resist the urge to run the heater for the first few miles until the engine is warm. Grill blocking helps too.

groar 11-01-2008 07:18 PM

Thanks for the replies. I will read this doc again.

With the scenic, a 2001 dCi (turbo diesel common-rail), I'm currently at EPA +33% in the morning when I have light traffic. In the evening with medium traffic I have "only" EPA +10%. With some short trips during week-ends, I have a current tank at EPA +20%.

With the megane, a 1997 turbo diesel, my best tank is EPA +50% and I don't have any iMPG display in it...

Whatever I'm doing, I can't do better then the EPA +33% during my morning commutes with the scenic. On the highway my best is Highway-EPA +10%, but to do that I have to drive at 10-15MPH under the speed limit. At steady Highway speed limit the Highway-EPA is simply unachievable...

Now my best tank with the megane was done this summer with hotter temperatures (20-30C vs 5-10C now) and lower traffic (25 minutes vs 35-55 minutes now). Against traffic, I avoid heavy traffic and stop the engine when I'm idle more than 15 seconds.

I have a big hill to climb. Thanks to the iMPG display I'm now accelerating quickly at the beginning and keeping a steady speed of 70 km/h to have a 7 l/100 consumption. Before I was accelerating more slowly and had a consumption over 10-12 l/100 during all the hill.

With the scenic I'm using both NICE-On Coasting and Dfco . I'm first using NICE-On Coasting to lower MPG without loosing too much speed, then using Dfco to have a null consumption and loose more speed. With the megane I was using only NICE-On Coasting so I was not saving as much fuel. Both cars have same tires with a 51 PSI sidewall. While megane is inflated at 46, scenic is inflated at 51 currently. But the comfort in the scenic lowered a lot from 46 to 51 so I may go back to 46. During my few city driving, I feel the coasting lasting forever

About grill blocks on both cars, I closed the lower one except the sixth toward the turbo.

I'm just wondering if there could be diesel specific tips, noticeably on these common rail diesels. These high pressure diesel have a better consumption, but seams to be more difficult to eco-drive.


ConnClark 11-01-2008 07:31 PM

As far as eco modding a diesel there are a few things you can do but don't expect a huge improvement.

Installing a free flowing muffler helps some by reducing back pressure. A free flowing muffler also helps reduce the amount of residual combustion gasses in the cylinders, this allows a greater quantity of excess air to fill the cylinder on the intake stroke. If you have a turbo diesel this also gives you slightly more boost which helps your engine run more efficiently and burn the fuel cleanly. It also helps the turbo to spool up quicker.

a nice article Browser Warning

A nice free flowing intake helps too. Also try and make sure the air you are drawing into the engine is cold. Some form of ram air induction helps too. My Mercedes Benz came stock with a cold ram air induction. The boys in Stuttgart did it for a reason.

another nice article Browser Warning

Note: do not use a K&N or similar filter. They do not filter as well as a paper filter. This allows dirt in the engine which can score the cylinder walls and drop your compression ratio. Low compression makes diesel run less efficiently and create more pollution.

The other thing to keep in mind is that if your injectors don't work right your engine won't burn all the fuel right. Keep your injectors clean and functioning properly. Sometimes improved injectors may become available and changing to these improved injectors will improve mileage and power while reducing both soot and NOx emissions.

That about does it for the easy modifications.

More advanced mods can be done but you have to know what your doing.

If you have a turbo engine adding an intercooler will help. It may shift your power band to a lower rpm to some degree but it improves both performance and mileage. It should also reduce NOx emissions.

Up grading to a more efficient turbo can help. This is not a straight forward procedure however and matching the performance curve to your engine can be tricky. A nice but very difficult upgrade is to go from a conventional turbo to a variable geometry turbo.

Duffman 11-01-2008 09:42 PM

ConnClark gives some good advice.

With respect to the muffler, if your car is a turbo diesel then remove the muffler, I did on mine, the turbo reduces a lot of noise by itself.

I run diesel conditioner in every tank. I do it for lubricity and water control but the extra cetane will boost MPG slightly.

My truck shuts off the fuel when decelerating in gear, if your car does that there may be times that in gear decel is better than clutch in coasting.

A synthetic oil like a 5w40 will do better than a 15w40 depending what your car takes, especially if you do more short trips.

A performance chip will give you a small increase as well if it is available for your car.

JQmile 11-02-2008 12:32 PM

When you drive, make sure not to be under boost at all. If the load is too high (say, accelerating up a hill) then the turbo will start to make boost, and the fueling curve will respond to the rise in boost pressure (add more fuel). This only applies on a turbodiesel of course. Oh and a timing bump will usually give a healthy increase in mpgs, 10-20% I'd say. That's why chips give you better mileage in newer computer controlled diesels, because they usually have more timing than the stockers. If you want to get nutty, you can also inject propane or cng in the intake's a tradeoff of about 2gal of cng to 1 gal of regular fuel, but if you can get propane or cng for cheap it is well worth it.

Piwoslaw 11-02-2008 03:51 PM


Originally Posted by groar (Post 70411)
I'm just wondering if there could be diesel specific tips, noticeably on these common rail diesels. These high pressure diesel have a better consumption, but seams to be more difficult to eco-drive.

I recently (1-2 months ago?) read about a small contraption for common-rail diesels. It ionizes the fuel with an electric shock, so the particals are much smaller when entering the cylinder and burn much more efficiently. The artical said that someone at some university built and tested it and it improved FE by 20%. It was easily retrofitted in two existing engines at a low cost. The actual shock required only a small amount of electricity, so the alternator hardly noticed it. It sounds really neat, almost too good to be true.

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