Tuning for economy

Nowadays, modern Common Rail diesel-engines are equipped with all kinds of anti-pollutant devices, eg. DPF, EGR. While these may bring pollutants down to a level that passes regulations, they inevitably also put more strain on the engine, and are also prone to break and cause problems with clogging etc. The software of the ECU is also designed to work with these devices, and the engineers have to find a compromise between pollutants, power and consumption. I have ditched the first one, and concentrated on the last two on my daily driver.

DISCLAIMER: Don’t take my words for granted, i do not have a degree in Automobile Engineering or diesel engine development. This is only my personal experience that i’ve gained on the subject in the last two years while studying the Bosch EDC16 ECU via trial and error. Engine tuning is also a very broad topic, so i’m not going to go deep into details.

While tuning for power on a EDC16-equipped BMW is relatively easy and information is easy to find online, there seems to be relatively big differences in ways that people tune for economy. Most popular tuners/companies don’t touch anything in the cruising speed area in the software maps, which is the area that you spend most of the time driving the car. This means they simply are not tuning for economy. Customers are nevertheless reporting a decrease in fuel consumption, but rarely with any hard evidence to support their claims. I have spent the last year trying out different setups to get my fuel consumption down, and i think i can say i’ve been somewhat successful.

In my opinion, there are a couple of things that make tuning for economy difficult.

  • 1. Not having access to a dynamometer
    • When making adjustments, the test conditions should always be exactly the same, so that any positive or negative change can be observed. The most accurate tests one can make without a dynamometer is driving the same route at same speed and in exactly the same weather conditions (temperature/wind/humidity). This is rarely achieved in real word. The problem is not that big when measuring power and performance, but when it comes down to identifying a 0.1-0.2ltr/100km difference in consumption, you really can’t tell if you can’t be sure that the testing conditions are 100% equal.
  • 2. Understanding the function principles of the engine
    • What happens when EGR (Exhaust Gas Recirculation) is turned off? Does the engine benefit from running with less boost on cruising-areas? Does the VE (Volumetric Efficiency) of the engine suffer from pumping losses generated by closed VNT on the turbocharger? One has to understand the functions of different parts.
  • 3. Everything affects everything
    • There is not a single adjustment to make the engine consume less fuel, but many small changes in the right places can make a big difference.

I have over 100 000km worth of average consumption data from my daily driver. The car is a BMW E91 320d (M47B20TÜ) 6-speed manual with approximately 380 000km on the clock and tuned to 208hp/497nm. The average consumption is calculated everytime i fill up, and can be easily tracked in a smartphone app.

As you can see, the all-time average consumption is 6.6 l/100km at the moment, while the last fill-up was 5.4 l/100km and all-time best 5.1 l/100km.Looking at all charted fuel-ups from the past three years, you can clearly see a downward trend during the last 12 months.

Earlier, no matter how i drove (urban or city-driving) the consumption was always between 6.5-6.9 l/100km. Now, it seems i’m averaging at about one liter less, 5.5-6.0 l/100km.

So, what are the modifications made to achieve this? Here you go:

  • Air/Boost
    • From the factory, lambda is >3 at cruise areas, which means that the amount of air going into the engine can be halved, and the engine will still be running lean and efficient.
    • Lowering boost helps to a certain amount, the theory behind this is simply that the engine is running “lighter” because of opened VNT-vanes on the turbocharger
    • With only lowered boost/too much lowered boost, the engine will perform well on a straight road, but response will be affected, since the turbo has to spool up from a lower pressure than before. This leads to noticeably higher fuel consumption during transient operation (eg. when accelerating to maintain speed up a hill).
    • Some tuners actually put more boost on cruise areas, i assume that’s not for economy but for better throttle response. With more air, the engine is able to inject more fuel instantly, not having to wait for the turbo to spool up.

Conclusion: Lower the amount of air/boost, but not too much, or it will affect things in a negative way.

  • Rail Pressure
    • Decreasing rail pressure makes the high pressure pump rotate easier (because of less pressure build-up), and therefore the engine should also rotate easier. The downside is that the duration of injection will be longer to maintain the same injection quantity. With less pressure, the spray from the injector nozzle will not be as good as with higher pressure.
    • Increasing rail pressure makes the high pressure pump rotate heavier, but the duration period will be shorter, which means a bigger bang (higher cylinder pressure) in the cylinder. The spray pattern of the injector / atomization of the fuel will also be better.

Conclusion: Raise the rail pressure on cruise areas up to 10-20%. The extra strain that the high pressure pump puts on the engine is easily defeated by the benefits of better and faster combustion.

  • SOI (Start Of Injection)
    • Advancing SOI is a sensitive topic, as over-advancing will destroy the engine really quick. The burning process in the cylinder is a very difficult thing to predict, and there are a lot of variables that comes into play when thinking about best SOI.
    • In areas where EGR normally is active (i assume EGR is turned off), there usually is a noticeable dip/valley in the SOI-maps. This dip can be smoothened out optimal consumption.
    • Furthermore, it is possible to add more SOI for the whole cruise area, but it really is guessing as long as one doesn’t have access to a cylinder pressure measurement device or a decent dynamometer.

Conclusion: Advance SOI in EGR areas (if EGR is turned off) to get rid of the dip in maps, if there are any. SOI can furthermore be safely advanced about 1-2 degrees at cruise areas, more than that will require a dynamometer or a cylinder pressure measurement device, if you don’t want to risk damaging your engine.

Your comments are welcome, feel free to agree or disagree 🙂