How to Eco-drive the VW PD TDI Diesel

[COcyclist]

I am starting this thread to pass on my experience driving my TDI Golf. I am the second owner of this car and purchased it when it was new enough that people on TDIclub said you could not get good mileage with the PD engine. (This engine has the high pressure fuel injectors built into the top of the head and driven by the cam.) I did a lot of reading and started doing some mods and my mileage started to improve. One of the great things about the TDI is that you can drive it like you stole it and still get nearly 40 mpg. (see the lowest entry in my fuel log)

The first key to driving a diesel efficiently is to buy the right car for its intended use. Diesels are great highway cars. This engine is most efficient when it is fully warmed up and it uses very little fuel, even at idle, so it warms up quite slowly, especially compared to a gasoline/petrol engine. It is not the best choice for short trips running errands around town. Buy a used Leaf for that. (Or better yet a bicycle). If your regular commute has a lot of stop and go traffic or city driving, a hybrid would be a better choice. If many of your trips can be an hour or more of open highway, you may love the TDI with a manual transmission. Mk IV automatics were not reliable, are heavier and less efficient.

Many of these tips will apply to the newer TDIs. These can be a relative bargain right now as "fixed" ones are showing up with extended factory warranties.

[COcyclist]

I will be double posting because the Ecomodder website will time out your token if you take too long typing. If you have not saved it, you will lose all your work. Also, I do not have a build thread so I will be mentioning some mods and how they relate to Eco-driving.

The first thing I did to improve mpg was to air up the tires. The original Goodyears are rated to 44 psi and I increased them to 50 psi. Later I backed off to 46 in the rears as they have less weight and forces acting on them. I am amazed how far this car coasts. Coast whenever it is practical. This is one of the easiest ways to improve your mpg.

The second thing that is a must is instrumentation. I purchased a ScanGuage II so I can monitor coolant temperatures, instant mpg, intake air temperatures and current and tank mpg. I wanted to be able to monitor engine temperature since I knew I would be blocking some of the radiator grill. VWs have a temperature guage in the dash but it is damped from the factory. In other words, it gets to normal and stays there. A ScanGuage or UltraGuage gives an accurate digital readout of engine temperature. This is vital to driving the TDI efficiently.

If you read about Hypermiling you may encounter some terms such as pulse and glide, DWL, EOC, DFCO and ICE-OFF. I will discuss these and how they relate to the TDI.

[COcyclist]

Pulse and Glide. As I stated in post #2 above- Coast when it is practical. If you are new to Ecomodder you may be amazed by the mpg numbers posted by some members on this site. Often the highest mileage is obtained by using Pulse and Glide. (Also know as EOC which stands for Engine-Off-Coasting.) Using this technique drivers accelerate to a target speed, say 60mph, and place the shifter in neutral and kill the engine. They coast or glide with the engine off to, say 50mph, and bump start the engine by putting it back in gear and letting out the clutch. This can be a way to get great numbers with a gasoline engine. DON'T DO THIS IN YOUR TDI! Ignore this at your own peril.

Here's why. #1- Diesel engines are very different from older gas/petrol engines. Gasoline engines have a throttle plate which does just what it sounds like it does. At part throttle it chokes off the intake air to maintain stochiometric ratio. This is good for combustion of gasoline but bad for low demand or idle fuel efficiency. In EOC Ecomodders install a kill switch near the shift lever to ground out the spark till the engine dies. They can glide with the engine off without turning the key. This avoids low load inefficiency and allows for glides using no fuel whatsoever. A diesel can burn at variable air/fuel ratios and are very efficient at idle. This means that there are much lower pumping losses at idle because there is no throttle plate and no vaccuum created and hardly any fuel is being injected. On my TDI I can coast (or glide) at 400mpg with the engine idling. There is no need to kill the engine to get good mpg numbers.

#2- Because the TDI shuts off the engine by not injecting fuel, it is much more complicated to kill the engine. I do not recommend using the key to do this. It not only shuts off the engine, it disables the airbags, vaccuum for power brakes goes down (the running engine turns a vacuum pump), no more power assist steering and it stops oil flow to the turbo. If your turbo has been running hard this is very bad! Don't do it.

[COcyclist]

In the TDI I use E-On Coasting because the Engine stays On and it seems to glide forever (for eons). Most of my trips are on 2 lane highways with other traffic so extreme Pulse and Glide would be rude at best and possibly dangerous with other cars around. I tend to drive at a steady state, usually a few mph below the posted speed limit if there are no other vehicles behind me. (Base Specific Fuel Consumption BSFC in this engine is around 1,800 rpm so driving closest to that rpm in 5th gear on flat roads is very efficient) However, I find I can get very good mpg numbers in rolling gentle hills and when coming into a town with lower speed limits by anticipating the road ahead and using an extended coast to the lower speed limits. If the road is wide open I will accelerate at the top of the hill and slip the gearshift into neutral and coast while getting between 350-400 mpg. This extends my coast times to at least 30 seconds or more which makes it more worthwhile.

DFCO- Deceleration Fuel Cut-Off. On the TDI the computer injects no fuel while engine braking. Mileage on the ScanGuage goes to 9999 or infinity. So why not do this all the time? The short answer is that it is going to slow the car down much faster than coasting in neutral. The trade off for using no fuel while engine braking is that you will use much more fuel in the long run, to get the car back up to speed. Should you use DFCO? Absolutely, but under the right circumstances. On long downhills where you would pick up too much speed? Yes! In fact I run the A/C full blast in the summer while engine braking to keep me cool and try to keep slowed down, or front and rear defrosters on high in winter. (Remember that part about diesels having no throttle plate? They don't engine brake the same as a gasoline engine. Now you know why semi trucks have a Jake Brake to create backpressure for engine braking) I use DFCO coming into stop signs and red lights. Otherwise, drive a little slower and coast, coast, coast on rolling terrain.

[COcyclist]

Some Ecomodders get good results with a gas/petrol engine by Driving With Load. You can use LOD on the Scanguage to monitor engine load and try to lock your foot in one position and drive with a constant load on the engine. This is sort of the opposite of what most cruise controls do. The factory cruise control will accelerate wildly uphill and at the crest go into engine braking. (Older gasoline engines do not have DFCO) This wastes fuel on the way up and kills your momentum, wasting fuel on the way down. In hills the use of cruise control is not the most efficient way to drive. DWL allows the driver to hypermile by anticipating the road ahead and allowing the car to slow somewhat on the way up and speed up (or E-On C) on the way down. In my experience with the TDI I have not seen significant mpg gains using DWL. The turbodiesel has tons of torque so even in a 1.9 4 cylinder most hills can be summited without a downshift. That said, it makes sense to use a modified version of this technique. If there are no other cars, I may slow slightly on the uphills if I anticipate that I will gain too much speed on the downhill. On rolling gentle hills I prefer to crest the hill at or slightly above the speed limit to extend my glide phase for as long as possible. This gives me better trip average mpg in my experience, and allows me to stay closer to the speed of other vehicles on the road. Here in the West the posted speed limit is 65 mph on rural highways but most of the traffic is driving 70 mph or higher.

[COcyclist]

Driving Without Brakes? There was a quote on this site a few years ago that said "If you are using your brakes you are doing it all wrong". Of course no one can drive without ever using the brakes and I want everyone to drive safely, first and foremost. The idea is to use the brakes sparingly. Someone here also said that when you use the brakes "you are turning fuel into brake dust". This means that if you come roaring into a stop you use heat friction to slow the car when you could have anticipated the stop and coasted and/or engine braked much sooner while using much less fuel. Therefore, roaring up to a stop wastes fuel. I have nearly 110,000 miles on my Golf and I am still on the original brake pads. They look nearly new and I live in the Rocky Mountains. Obviously I use the brakes as little as possible and I do not have to deal with stop and go traffic very often.

[GreenTDI]

Nice thread, COcyclist!

I also have some experience with TDI's. Learned to drive in a VW Transporter 2.5 TDI 5 cilinder, and now I have my own car with a 1.2 TDI 3 cilinder.

Big difference between the engines (big vs small) is that pulse and glide in the 2.5 TDI wasn't very effective, the engine had great breaking power because of internal friction, and coasting was possible but only at low speeds or steep descents because of the car's dramatic aerodynamic drag - it was a van.

The small TDI however has very little braking power. In fact, at small descents (2-3%) the car won't lose speed at all when releasing throttle. At decents of 5% and more it will even accelerate and I have to use the AC and/or downshift to generate more braking.

But I'm not using pulse and glide or coasting to get good mileage (yet).
On flat roads I will use cruise control and maintain steady speeds.
In hilly landschapes I'm never using cruise control, but I'm guided by the slopes and descents. So downhill I will gain some speed to maintain it below, and lose some speed going back uphill. For example, down the hill I'm speeding at 75 mph, and I'll reach the top at 55 mph. Always staying in highest gear, as you said, the TDI has a lot of torque and won't force you to downshift.

[COcyclist]

Originally Posted by GreenTDI

coasting was possible but only at low speeds or steep descents because of the car's dramatic aerodynamic drag - it was a van.

I agree. I have a Dodge Caravan and I just drove down a hill that I can coast for over a mile in the VW. In the van I had to drive down in gear and could only coast briefly in neutral on the steepest sections.

Instrumentation is helpful to learn the balance between coasting and using engine braking in the VW.

[COcyclist]

As GreenTDI stated, cruise control can be very useful on flat roads. I find it can be a very good way to achieve higher mpg results under the right conditions. Ideally, cruise control will be used on straight flat roads with light traffic. You don't want to come speeding up onto slower traffic forcing you to kill momentum by braking. Also, even though the TDI gets great mileage at 50 mph you don't want to create traffic backups behind you and encourage drivers to take crazy chances to get around if the speed limit is 65 mph. Safety for all is more important than a few mpg for a Hypermiler.

[ptitviet]

Yup happy to share my observations (with my '15 Octavia 3 1.6TDI Greenline Combi). I do not use pulse and glide a lot on my car, as it tends to be less useful when the engine is not too powerful (my former car was a 265hp Renault Megane RS and I could get almost 2l/100km less by using pulse and glide). I only do hill pulse and glide by pulsing up and gliding down. When coasting I switch AC off as it the engine is less efficient when idling.

No mods on my car, only folding passenger mirror on long highway trips (0.1l/100km better). I only use ECO official tyre pressure, as I live in the mountain and want to keep the best handling possible. And when on eco mode, I drive only 100/110km/h on highway instead of 120/130 (Switzerland/France speed limits).

Until now I have owned the car one winter and got 48mpg (4.9l/100km) which is not bad with short trips uphill to the ski resorts and winter tyres. I am hoping to meet 3l/100km mark this summer with long holiday trips on secondary roads (80-90km/h). With winter tyres and longer trips (30km) I can already get about 4L/100km (58mpg), thanks also to the very good aerodynamics (0.59m2 measured by me thanks to Matlab).

I'd be happy to try front grille cover, but I am not very good with my hands and do not want to put something with no good finish on the car.