Hybrids and oil: does the engine really hardly ever run?

Sellers of used hybrids are only too happy to tell you the same story: the car, they say, runs mostly on electricity, the petrol engine only “helps out” now and then, so the oil in it can stay in twice as long. It sounds logical, and there is no shortage of buyers who believe it. But the oil in such an engine does not age any more slowly. As a rule, quite the opposite.

Yes, a hybrid’s engine really does put in fewer hours than the engine of an ordinary car with the same mileage. Only oil is not worn out by the kilometres on the odometer: what “wears” it is cold starts, condensation, fuel in the sump and simply time. And that is exactly where the hybrid is worse off.

How a hybrid’s engine works

You start an ordinary engine and it runs until you switch it off: within a few kilometres it has warmed up, and from then on the oil is hot the whole time. In a hybrid (it makes no difference whether it is a full hybrid or one you charge from a socket) the engine is run by the electronics, and they have one job — to burn as little fuel as possible. Pulling away, in a traffic jam, at low speed the car is often moved by the electric motor while the petrol engine is switched off. More power needed, or the battery has run down — the engine starts, runs for a few minutes and stops again. (In a so-called “mild” hybrid the engine runs almost as it does in an ordinary car with stop-start, so everything below applies above all to full hybrids and plug-in hybrids.)

So on a trip around town a hybrid’s engine starts and stops again and again, and quite often it simply never gets the chance to warm up properly. It is like a kettle that keeps being switched on and then switched off a minute before it boils — and you never get any boiling water. For the oil, a hybrid driven mostly in town and over short distances means those same “severe operating conditions”, only in an even harsher version. They have already been covered in the article on which factors shorten the service interval: irregular driving, short trips on which the engine has no time to warm up, traffic jams. A hybrid in town collects the full set at once.

A plug-in hybrid has its own extreme. If you charge it at home every day and do not go far, the petrol engine may not start for weeks. Meanwhile the oil just sits there together with the condensation that has formed in the sump, and then the engine starts — and once again only for a short while.

It is no accident that additive makers test oils for hybrid engines under separate, tougher programmes. But “for hybrids” on the can is no substitute for the car manufacturer’s approval.

Water and fuel in the oil

Burning petrol produces a good deal of water vapour. Most of it goes out through the exhaust pipe, but some of it gets into the sump together with the gases that blow past the piston rings. While the engine is hot, the crankcase ventilation carries that vapour back to the intake, and from there through the cylinders into the exhaust. In a cold engine it condenses on the cold walls and ends up in the oil, and once mixed with combustion products it turns into a rather effective acid. The same condensation that threatens a car left standing for long periods builds up in a hybrid even when it is driven every day.

Fuel is much the same story. On cold cylinder walls some of the petrol has no time to burn, runs down past the rings into the sump and thins the oil. In a warmed-up engine the light fractions of the fuel then evaporate, but in an engine that never warms up they simply stay put. Water and oil make an emulsion, fuel thins it: the oil oxidises faster, lubricates worse, and the engine’s parts begin to rust.

Add bioethanol to that: in Ukraine the law requires petrol (except 98-octane and above) to contain more than 7% bioethanol, and alcohol readily draws in water. More on fuel and alcohol in the oil in the article on oil change intervals today.

What the dipstick shows

Some of this can be seen without any laboratory. Take the trouble now and then to pull out the dipstick and look under the oil filler cap. Only first switch the hybrid system off completely (the READY light must go out): a hybrid’s engine is quite capable of starting by itself, without warning.

1. A whitish, mayonnaise-like deposit under the filler cap or on the dipstick is water. In winter, after short trips, a little of it under the cap turns up even on a healthy engine: it is the same condensation. But if the “mayonnaise” does not go away even after a long run on the motorway, and the coolant level in the expansion tank is slowly dropping, do not wait for the oil change — take the car to an experienced mechanic. Most likely it is already antifreeze in the oil.

2. The oil level rises between changes. Do not smile — it does happen that there is more oil on the dipstick than there was after the change. Oil does not breed on its own: so fuel and water have got into the sump.

3. The dipstick smells of petrol. One more sign that fuel is collecting in the oil and has no time to evaporate.

The colour of the oil, on the other hand, tells you nothing, as always: dark oil is not necessarily old, and light oil is not necessarily still fit for use.

Which oil and how often

So how often should you change the oil in a hybrid, you will ask? The change interval in the service book, as a rule, has two limits: so many thousand kilometres or so many months — whichever comes first. And cars that work out the interval themselves take into account not only mileage but also engine speed, temperature, fuel burnt and time. A car that is driven a lot usually runs out of kilometres first; a car that does little driving in town runs out of months. And remember that the odometer also counts the kilometres the hybrid has covered on electricity. So change the oil at whichever limit comes first, and do not stretch the interval just because “the engine hardly ever ran”.

A useful habit: at least now and then let the engine warm up properly — on a trip out of town, not to the supermarket and back. In a hot engine a good part of the water and the light fractions of fuel evaporate from the oil. And owners of plug-in hybrids would do well to select, for such a trip, the mode in which the petrol engine runs: with a full battery the car can go out of town and back without starting it once.

Now about the oil itself. Hybrids, Japanese ones especially, are often designed for very thin oils: some Toyota hybrids are filled with 0W-16 at the factory, and some even with 0W-8 to the Japanese JASO GLV-1 standard. Why these engines need such oils, and no others do, is explained in the article on ultra-thin 0W-16 and 0W-8 oils.

Some people, finding the oil on the dipstick thin to the touch, decide the manufacturer has made a mistake and put in something thicker just in case. I will say it once more: the viscosity of the oil must meet the car manufacturer’s requirements, and a hybrid is no exception. A thicker oil will drive neither water nor fuel out of the engine — it will only take longer to reach the rubbing parts in those first seconds after each of the many starts, when the engine needs lubrication most. And of course, apart from the oil, you need to pour or tip NOTHING into the engine — neither “against water” nor “against wear”.

So, the oil in a hybrid is not “resting”; it is working in conditions that, as a rule, are harder on it than in an ordinary car. Change it no less often than the service book says, and if the car lives in town, more often still. Leave the stories about an engine that “hardly ever ran” to the used-car sellers.

Further reading:

• How often to change the oil?
• Why is town driving severe conditions for an engine?