Can short journeys damage my engine?

QUESTION

Hi John,

I'm a huge fan and really enjoy listening and learning from your discussions. The question I have is essentially to do with short trips.

During the week, I have a short commute, in the region of three miles each way, which is not quite enough to get a vehicle up to temperature, and certainly not enough to prevent any dilution of oil with fuel, I'd presume.

My dilemma is with my diesel 2013 BMW 320d (with a DPF, EGR etc), a BMW Z4 E85 with the 3.0-litre N52 straight-six engine, and a Land Rover Defender with a Ford 2.4 Transit diesel engine (with not EGR/DPF).

I guess I'm a glutton for punishment, but happy with my individual form of sadomasochism.

Perceived 'wisdom', shall I call it, has it that my best bet is to use the petrol for the short journeys. But every weekend I'll take the 320d for a relatively long drive anyway which would heat-up the DPF.

Other than the dilution effect and possible clogging of the DPF - if this is even a concern - is there really any merit to the potential myth of petrol short, diesel long journey 'wisdom'?

I've never fully grasped the 'short journeys do more damage' idea. Over an equal millage, I presume it is true, with a greater proportion of cold starts and stop-start traffic sections. But there isn't a magical healing property of a long trip for bearings and other metal on metal interactions, I presume, and wear does still occur on long trips, but at a reduced rate.

I'd love to hear your thoughts on this, and if there is any merit to it, especially given how much better oils are continually getting.

All the very best,

Peter

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ANSWER

Hey Peter,

Okay, three miles Monday-to-Friday, comprising 10 cold starts, is not that much driving, so not that much dilution of the oil. A weekly long drive should clear that up; but depending on your lockdown situation you might need to stay in the vehicle and make no stops.

Get the oil changed twice as often as the manual recommends if this is a concern. (Pretty cheap hedge.)

Essentially, you’re trying to heat the oil so that the volatile components contaminating it evaporate off (mostly fuel and water). The DPF plays no role in this. It ‘heats up’ when a pressure sensor reads that it’s getting clogged, and needs to ‘regenerate’. At this time, additional fuel is injected into it, and it turns into a furnace.

As for short journeys doing more damage: The parts (like rings and pistons) don’t fit properly when cold. Fuel and water blows past them into the sump, contaminating the oil. (Water is a principal combustion product, so fuel + O2 becomes CO2 + H2O. The water is produced as a gas.)

If you drive for a long time, the oil heats up and the water and fuel evaporate out. If you shut down early, it doesn’t. When everything goes cold again, from shutting down, that heat condenses (like overnight, for instance) and it becomes even more water. Have a look at your bonnet after a cold night and you might see a thin patch of condensation - that’s happening inside your engine.

Wear rates are high when cold, because the oil’s not especially thin at low temps (it’s very viscous), and the parts don’t fit properly. (Metal expands when it heats up, and the design clearances take this into account.)

JC

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