Why your Turbo Diesel is not so good in the Winter

[Welly]

Courtesy of 'BMWLand' forum, discussing why Turbo Diesels with Intercoolers are not so good in the Winter, contrary to popular thinking......

When I was running the 115 PD TDI Passat, I had a pretty good idea of consumption for most runs I did. One trip up from Wiltshire the ambient temperature was -3°C, until I left Tebay after breakfast. Normally I’d be well over 60mpg by this point. But this particular morning, no way would the consumption move much above 55mpg, even though the speed and other conditions were as I’d drive many times. I’d always used the same fuel, fuelling at the same BP garage.

Once the ambient temperature was climbing, the consumption improved, I arrived home with an accumulated figure indicating 57.5mpg. 60.9mpg from Tebay to home.

I started researching the subject to see if there was an explanation. Was it ice crystals? Or something else? But as the air is compressed I couldn’t imagine the air was still sub zero on entering the engine.

Started with the ‘cold air’ thinking.

It's not really COLDER air that makes more power. It's actually the DENSER air that does. It just so happens that colder air IS denser.

Then purely by chance, time wise, Mike Fishwick who writes quite regularly in Volkswagen Driver on his exploits in VW Golf’s, (also runs a BMW) mentioned his observations that the intercooler in his Golf TDI was ‘too efficient’ and his economy was suffering at low ambient temperatures.

He had experimented with partially covering the intercooler with a piece of card and gave some stats’ on how much to cover, how it would bring the economy back to normal at different ambient temperatures. Incidentally he was still covering part of the cooler at temperatures towards 12 - 15°C. The temperatures where both my son and I found the diesel TDI’s gave their best economy. Coincidence? I don’t think so.

So I went out to get some more info. The guys who run large rigs in the States, particularly through the mountains in wintertime, had some interesting observations. These guys are serious diesel nuts. Watching IAT, EGT & boost gauges as they run. One of their more technical guys clearly spelled out, the intercooler could be ‘too efficient’ and power would drop and engines smoke as ambient’s dropped.

Summing it up, I wrote these notes sometime back…

“We can actually lose power if the air gets too cold because the engine can't heat it enough to burn all the fuel. We want cool air, but not ICE COLD air going into the engine. (I’ve always understood we want cold air for petrol, as it assists efficiency. Hot air preferred on the warm up cycle. Hence the automatic flaps on the air pipes).

One reason the intercooler was added to diesel engines, was to assist emissions. It lowers the temperature of combustion and along with exhaust gas recirculation (EGR), subsequently reduces the formation of NOx.

We have a compression ignition engine. It is the heat generated in the charge air in the cylinder during the compression process which ignites the fuel. The final temperature of the compressed air is a mathematical function of (among other factors) the initial temperature at the beginning of compression (in effect, IAT) and compression ratio. Under extreme conditions, IAT can get so low that the temperature of the compressed air when the fuel is injected is too low to support efficient combustion. Therefore, there is a limit regarding how low one would want IAT to go.

I understand that large industrial diesels that are sold for extremely cold climates often have a second heating coil in each liquid-to-air intercooler for just this reason - to keep the inlet air manifold temperatures from getting too cold.

At higher air flow levels, it's possible that the charge air cooler becomes such a restriction that the disadvantage of this restriction outweighs the benefits of the cooling effect it has. Some trucks run without such a cooler for that reason.”


So my understanding… there is a temperature where the engine may not really be hot enough ‘internally’ in certain conditions, typically in my case at the light load required at a 70 - 75mph cruising speed. Also at such good consumption figures the measurable difference is so much easier to see.

At a constant 70mph on a still, warm day, the Passat would show almost 70mpg on a flat piece of motorway. You can see how wind, electrical load, A/C, etc., would soon show a marked change in consumption.

Interesting, I had never really thouht about this before but it does make sense. I might try blocking off part of my Intercooler and see what goes, I am sure my HDi is faster in the Summer, with less smoke too.

[jameslxdt]

diesel engines work far better with a high combustion temperature than cold, hot air is what burns the fuel, so the hotter the incomming air, the better combustion and power, the only reason an intercooler is fitted is to avoid detonation

[Welly]

Well, yesterday I fashioned an intercooler sheild to block half of it off, but then chickened out and put it all back to how it was