Low oil consumption

[rverdoold]

We all know that subie boxers consume some oil. My gen 1 legacy used about half a litre (quarter) per 2000km (1600 miles).

However my 1.6 impreza on propane hasn't used any in 10000km (6000 miles) or nearly none. It only ran no 70k km mainly highway. The dealer replaced oil last time and said he used high quality semi synthetic oil.

Could not consuming oil be bad??

[Fairtax4me]

You will always have a small amount of consumption. If there were NO consumption the rings would wear out in nothing flat. Low consumption is great. Less oil in the cylinder creates a more stable fuel burn, giving more power and better mileage (more efficient). It is using a bit, but just not enough to notice on the stick, which is really the ideal rate of consumption. I wouldn't worry about it.

[nipper]

The reasons car use oil is that it is vaporized at high cylinder temps. It is sprayed on the bottom of the pistons to cool them. Running propane ( I forget the mechanics) it seems your pistons arent getting as hot as they would with gas, so the oil never really vaporizes.

 

 

Just be happy

 

 

nipper

[OB99W]

There are numerous ways that oil can be consumed in an internal combustion engine. However, if we assume that the only difference between the two engines is the fuel they're being run on, then one possible explanation has to do with fuel dilution of the oil. Some gasoline (especially under cold-start conditions) gets past the rings and winds up mixed with the oil in the crankcase. That reduces the oil viscosity, lessening its sealing capabilities and making it more likely that oil will then be burned. Consumption due to that cause tends to get progressively worse.

 

On the other hand, should propane get past the rings to the crankcase, it won't dilute the oil to any significant extent. Its fumes will be removed by the PCV system, and the oil's viscosity won't be materially affected.

[rverdoold]

The system is a Bi-fuel so the cold start is always on 95 octane gasoline (unless i force it to start on propane). After 2, 3 minutes the system switches to LPG. This switching is based on RPM, coolant temp and some other variables. So when engine is warm it starts on gasoline and then switches in few seconds. Propane is injected on top of the intake valve on the intake manifold by piezoelectric injectors. The time of injection is crucial and controlled by a second ECU which stops the gasoline part of the car-ECU.

In general the burn of LPG is hotter than the gasoline burn, therefore valves and valve rings (seats) have to be harder or will just wear quicker. It is advised to not pass 4k RPM.

Last week i was for a 1000 mile trip in Germany and drove 90 to 100 m/h for 2 hours (then stop and refuel LPG not that tank was empty but just a rest). The consumption of LPG then was approx 22 MPG, (at 55 m/h it is 30 on LPG and 34 on gasoline). I love the LPG because it is very cheap in europe (around 5 eurocents per km).

 

It burns very clean I never pull full through the RPMs and drive efficient perhaps this also influences the oil consumption. I checked again this morning it was about 2 tens of the second hole (dividing the distance between holes in 10 parts). This should be arround 200 ml over 10000 km.

[Scoobywagon]

Something else to consider here would be the state of your propane at the point of injection. Are you drawing liquid or vapor from the tank? If you are drawing liquid, then overall operating temp should be just a bit lower.

 

Propane's peak flame temp is about 3614*F as compared to gasoline's 3591*F. However, it is possible for operating temps to be lower for a couple of reasons.

 

First, if you're injecting the propane in liquid form, then the first thing its going to do when exposed to the high temp of a cylinder is flash back to a gas. Doing so also absorbs huge amounts of heat. This is one of the reasons why propane makes a pretty darn good refrigerant.

 

The second reason for lower overall operating temperature would be because propane burns VERY quickly. The rapid burn rate limits the amount of heat absorbed by the cylinder walls and piston. Essentially, you get a big spike in cylinder temp, but the cylinder wall temps don't come up very much because the heat source goes away so quickly. That's why you can run your hand through the flame of a candle without being burned. Its not that your hand doesn't absorb any of the candle's heat, just that the limited exposure means heat absorption remains with in safe tolerances. Essentially, you get a higher peak temperature, but a lower duration. This also means the cooling system has a better chance of maintaining control of engine temps.

[rverdoold]

wow, did not know propane could be used as refrigerant. Always love technical explanations .

The LPG is in liquid form in the tank and also drawn as a liquid, but before injection it is changed to gaseous state. Since each cylinder has its own injector it is injected directly above the valve. This reduces the change of backfires. This can occur when the LPG is injected as single point injection or even worse as carburettor system.

I looked up the manufacturer of the system and they say it eliminates the oil dilution by gasoline and no more wash down on the cylinder walls. This would then explain the lower oil consumption

[OB99W]

wow, did not know propane could be used as refrigerant. Always love technical explanations .

Certain hydrocarbons are pretty efficient as refrigerants. (Google ''R-290a''.) The debate is about how safe they are for the application. A ''replacement'' for R-12 (a CFC) is HC-12, which is about 78% propane and 22% butane with possible variations in the ratio and inclusion of other gases such as isobutane, etc. By the way, it wouldn't surprise me if your fuel contains some butane, rather than it being pure propane.

 

 

I looked up the manufacturer of the system and they say it eliminates the oil dilution by gasoline and no more wash down on the cylinder walls. This would then explain the lower oil consumption

I'm glad that the manufacturer agrees with me.

[WoodsWagon]

I'd also argue that the subaru engines aren't "known" for eating oil. If the engine isn't beat and the PCV system is in good shape, it shouldn't use a noticable amount of oil between oil changes. The 3 subarus in my family go 5k miles between oil changes, and unless they're leaking oil, the level on the dipstick hasn't gone down between oil changes. Loosing a quart every 3k is excessive in my book.

 

I had an EJ22 I'd hurt bad, I had freshly rebuilt it and drove it cross country the next week. The hood latch cable broke part way across, so by the time I got to my destination it was down to under a quart of oil left in the pan. When the rings are seating it's normal to use some oil. Unfortunately I seated the rings and ran them out of oil all at the same time.. It used oil ever after that, and would burn exhaust valves due to the carbon deposits. The last few months I ran it I put a quart or more a week through it, with me commuting 300 miles a week. That was way bad. I actually had oil slime stuck to the back of the car due to the amount running out the exhaust. I'd just pull the plug on the injector of the dead cylinder and keep going.