As the weather cools down, the plucky TDIs will continue to forge ahead. However, they will do so with slightly damped performance and economy. The reason for this is mystic at best. However, many website forums out there point to winter blends in diesel fuel, for instance. I will take a different approach and blame air density.
When the air is cooler, there is less performance and air mixture to be had in each cylinder. So as the TDI puffs along, the cold air makes for a slightly more difficult compression and adversely affects ignition.
That pinging noise you hear? Well, that’s an example of poor ignition. That sound means that the compression in the cylinder is actually igniting the fuel before the cylinder has made complete compression. In TDI engines, the pinging is greatly reduced because the piezo injectors catch this subtle pre-ignition and inject (squirt) more diesel fuel into the cylinder just at the pinging occurs. This effectively maximizes the compression opportunity in the engine, but also uses more fuel.
So don’t quite think you can’t rev the TDI up past 3k rpm on a normal drive during a cold day. What you should think about doing is starting up the TDI from a dead cold start and begin to drive away immediately. By diving off as soon as possible, you are increasing the load on the engine and are ultimately giving it a better chance to reach a normal operating temperature. If you start the TDI on a cold morning and let it sit there and idle for several minutes, you are simply using up more fuel without generating additional heat to warm up the car.
Interestingly, however, is this article from AA, the British version of AAA. The article explains that DPF regens are less likely to occur in cars that don’t get much use. This, in theory, means that if a TDI has a short commute in cold weather, it is not likely going to reach the optimum temperature for a DPF regen. Furthermore, it also explains that driving along in 6th gear on the highway to keep the RPMs low, might not be a good idea.
Without a load on the engine to warm it up, and with extra fuel being used to correct pre-ignition, you must also be cautious of driving at too low RPM’s as DPF regens might occur more frequently, which all leads to more fuel consumption.
So, in conclusion, it will be a good idea this winter to drive your TDI a little sooner and a bit more aggressively to achieve better MPGs. That sounds like a win/win to any TDI owner!
