Diesel Engines - Cold Start Oil Concerns?

[Blue_Angel]

Below is my understanding - if incorrect please correct me.

Gas engines run rich when they're cold, which can lead to fuel contamination/dilution of the oil. This is why its recommended to drive a gas car gently to warm it up instead of letting it idle; warms up quicker, produces fewer emissions, and less oil washing of cylinder walls and fuel contamination in the oil.

It's also important to get the oil in a gas car HOT on a regular basis to make sure to evaporate fuel and moisture contamination out of the oil, which can be a challenge in cold climates and where shorter trips are typical.

Diesel engines always run lean, even on cold starts, because they have no catalytic converter to warm up. Does this mean fuel dilution with diesel engines is less of a problem (cold starts/idling) than with gas engines, and is getting the oil hot to burn off contaminants is less crucial? I understand that there can still be moisture contamination in the oil, but is it any more or less of an issue than with a gas engine?

Because a gas engine runs rich when cold, and in winter the engine spends more time running cold than in summer, is the winter impact on a diesel's fuel efficiency expected to be less than that of a gas car?

 

[DemoFly]

Modern injected gas engines don't run rich when cold. They run an enriched fuel map for colder intake air temperatures and cylinder temperatures. Colder air is more dense, so it needs more fuel for stoichiometric fuel ratios. If extra fuel were to not be added, there would be an excess of oxygen and the engine would be running lean. Oxygen sensors are self-heated now, and vehicles go into closed loop fueling very quickly.

Diesel engines don't always run lean. But they do run lean when they can, and especially at idle. Fuel dilution is still a thing that happens in diesel engines. It's not anything to be worried about for most use cases and it's aided by large oil capacities.

Here is a resource on fuel dilution in diesel engines. The section on Wet Stacking is very informative.

Fuel Dilution in Diesel Engines | An In-depth Explanation

Information on fuel dilution in diesel engines, including an in-depth explanation of the concept of wet stacking, the importance of adhering to the proper maintenance schedule, and many methods to reduce the rate of fuel dilution. Fuel dilution is the normal process by which diesel fuel enters...

www.dieselhub.com

In my experience, the loss in efficiency between gas and diesel vehicles in the winter is about the same.

 

[Accent Abuser]

Below is my understanding - if incorrect please correct me.

Gas engines run rich when they're cold, which can lead to fuel contamination/dilution of the oil. This is why its recommended to drive a gas car gently to warm it up instead of letting it idle; warms up quicker, produces fewer emissions, and less oil washing of cylinder walls and fuel contamination in the oil.

It's also important to get the oil in a gas car HOT on a regular basis to make sure to evaporate fuel and moisture contamination out of the oil, which can be a challenge in cold climates and where shorter trips are typical.

Diesel engines always run lean, even on cold starts, because they have no catalytic converter to warm up. Does this mean fuel dilution with diesel engines is less of a problem (cold starts/idling) than with gas engines, and is getting the oil hot to burn off contaminants is less crucial? I understand that there can still be moisture contamination in the oil, but is it any more or less of an issue than with a gas engine?

Because a gas engine runs rich when cold, and in winter the engine spends more time running cold than in summer, is the winter impact on a diesel's fuel efficiency expected to be less than that of a gas car?

Most cold start wear is due to internal part clearances being way too large (engines are obviously designed for operating temperature tolerances), the lack of heat needed to replace wearing tribofilms such as zddp to phosphate glass, and moisture, which causes corrosion. You should drive an engine gently when cold because the parts are literally banging around because they're too loose and there is a limited sacrificial layer of additives that can no longer be replaced until the temperature is hot enough.

 

[ka9mnx]

When I was in the biz we installed diesel fuel heaters in class 8 trucks that traveled in Northern climates, especially Canada and Alaska. They were mostly installed to improve fuel efficiency but also to prevent fuel gelling. Truckers loved them.

They also used block heaters which greatly improved cold starting.

 

[ironman_gq]

You're correct on diesels running lean but wrong on your reasoning. Diesels do not have a throttle plate like a gas engine, they regulate power output by directly varying the amount of fuel injected. More fuel, more power as long as there is enough air to support an efficient burn. Black smoke is poor combustion, white smoke is unburned fuel (in an otherwise healthy engine). In cold temps diesel engines have a harder time getting complete or efficient combustion until the cylinders warm up, it's common for some black/white smoke on startup and increased fuel contamination in the oil. I wouldn't say it's any worse than a gas engine but it still happens.

 

[UG_Passat]

My old TDI had a catalytic converter... back in 2002.

With diesels, there really isn't a lean condition. The fuel uses up the air it needs to burn. But there is a thin line of looking clean and visible smoke. So normally, there is an excess of air in the combustion chamber, that is used to mask the smoke. But with mods increasing the fuel injected, it can cross into the line of coal rolling.

Back then with an relatively uncomplicated diesel engine, we would have the nozzles swapped to inject more fuel, which there is a nozzle orifice line to cross for proper atomization of the diesel fuel.

Of course, ECU programs, up the boost, and thus up the fuel injected also.

I was coal rolling people before it was a term.... pitied this one jerk guy flashing his lights at me when traffic slowed down to a crawl and wanted me to get out of the left lane. As soon as traffic opened up, I smashed the gas pedal to the floor so he can choke on my smoke in his open top convertible.

 

[BMWTurboDzl]

Diesels run a much higher compression ratios so there's always going to be some blow-by and soot especially at cold start.

Modern diesels DO have to heat up the SCR-catalyst as quickly as possible and they usually do that by closing the EGR valve.

As far as efficiency is concerned. Artic (aka winter) diesel contains less BTUs vs summer formula so there's a slight FE hit.

 

[edyvw]

Diesels run a much higher compression ratios so there's always going to be some blow-by and soot especially at cold start.

Modern diesels DO have to heat up the SCR-catalyst as quickly as possible and they usually do that by closing the EGR valve.

As far as efficiency is concerned. Artic (aka winter) diesel contains less BTUs vs summer formula so there's a slight FE hit.

BMW has transmissions programmed to shift later when cold to warm up SCR as quickly as possible, unless shifting manually.

 

[BMWTurboDzl]

BMW has transmissions programmed to shift later when cold to warm up SCR as quickly as possible, unless shifting manually.

Ya. I remember that with my 335d. It was annoying

 

[Snagglefoot]

 

[edyvw]

Ya. I remember that with my 335d. It was annoying

But it had electric heater for HVAC. Something A LOT of manufacturers that have issues warming up engine could learn from.

 

[sk_pete]

It's also important to get the oil in a gas car HOT on a regular basis to make sure to evaporate fuel and moisture contamination out of the oil, which can be a challenge in cold climates and where shorter trips are typical.

100-400´f
impossible to get rid off completely; unless you run the oil above 400´f
i guess nobody does

 

[Blue_Angel]

BMW has transmissions programmed to shift later when cold to warm up SCR as quickly as possible, unless shifting manually.

Ha, that must be it! I've noticed this 535d hangs onto gears longer than seems necessary for the first minute or so of driving. Funny, my Wife's 323i (gas 2.5L six) does this too, though I'm not sure if it's emissions related or just oddball programming... the transmission logic in that car is the one thing that's disappointing about it.

 

[Blue_Angel]

You're correct on diesels running lean but wrong on your reasoning. Diesels do not have a throttle plate like a gas engine, they regulate power output by directly varying the amount of fuel injected. More fuel, more power as long as there is enough air to support an efficient burn.

Yes, this I'm aware of and is why they're always lean - the throttle in a diesel is restricting/metering fuel to the engine, not air.

Funny enough, my 535d does actually have a throttle body, but if I had to guess it's related more to controlling flow through the EGR circuit than throttling the engine.

 

[Blue_Angel]

100-400´f
impossible to get rid off completely; unless you run the oil above 400´f
i guess nobody does

Excellent data, thanks! So I can get moisture out of the oil since it regularly hits 100C oil temps, but ya, not even going to be close to burning off any diesel.

 

[Blue_Angel]

But it had electric heater for HVAC. Something A LOT of manufacturers that have issues warming up engine could learn from.

I'm pretty sure my 535d has an auxiliary heater in the HVAC - it's blowing warm air a couple minutes after a cold start. I'm impressed with how quickly this engine heats up, but there's no way the coolant is delivering that kind of heat from the heater core that quickly.

 

[Blue_Angel]

As far as efficiency is concerned. Artic (aka winter) diesel contains less BTUs vs summer formula so there's a slight FE hit.

I forgot about that - thanks for the reminder! Funny, since I get better mileage with my narrower winter tires than I seem to with my wide summer tires, LOL. Firestone Indy 500s are not a LRR design!

 

[Blue_Angel]

Here is a resource on fuel dilution in diesel engines. The section on Wet Stacking is very informative.

Fuel Dilution in Diesel Engines | An In-depth Explanation

Information on fuel dilution in diesel engines, including an in-depth explanation of the concept of wet stacking, the importance of adhering to the proper maintenance schedule, and many methods to reduce the rate of fuel dilution. Fuel dilution is the normal process by which diesel fuel enters...

www.dieselhub.com

Excellent article, thanks for sharing!

I do wonder about one thing they say: "...it should not be allowed to idle for extended periods of time; 3 to 5 minutes (depending on ambient temperature) of idling is generally sufficient to allow oil pressure to stabilize and engine temperature to reach a suitable range before driving in cold conditions. In warmer weather, 1 to 2 minutes of idle before driving is entirely sufficient."

This is at odds with everything I've seen with gas engines, which usually achieve stable oil pressure in a matter of seconds. Unless they're talking about large industrial diesel engines? But they are talking about driving, so heavy truck engines maybe? I see no reason why a passenger car/truck diesel engine would take any longer to circulate oil than a gas engine would. Heck, my 535d takes 0W-30!

 

[Popsy]

Yes, this I'm aware of and is why they're always lean - the throttle in a diesel is restricting/metering fuel to the engine, not air.

Funny enough, my 535d does actually have a throttle body, but if I had to guess it's related more to controlling flow through the EGR circuit than throttling the engine.

In some case it's also for turning off the engine in a more graceful way than just cutting the fuel supply.

 

[ironman_gq]

Yes, this I'm aware of and is why they're always lean - the throttle in a diesel is restricting/metering fuel to the engine, not air.

Funny enough, my 535d does actually have a throttle body, but if I had to guess it's related more to controlling flow through the EGR circuit than throttling the engine.

Works with the EGR to control flow and is sometimes called an anti-shudder valve. Stops that little bit of run on that diesels tend to have.