Oil vs Coolant Temperature

Question

Could an engine be run dry, as is the case with solid blocks, meaning no coolant, if a sufficient oil cooler were to be installed?

Essentially, if we could keep the oil temperature between 210 and 240f, would the cooling system (radiator, etc) become unnecessary; as the oil operating temperature is correct? Could we not simplify things by using a much larger oil capacity with a sufficiently sized cooler, in turn, eliminating the 'water cooling' system?

Isn't the oil temperature the only truly critical factor, as it is the lubricant protecting the engine. I'm trying to understand why we use water cooling when we could simply cool the oil instead.

Answer 1

I suppose you could, but there's several drawbacks to doing so (might be more, these are just off the top of my head):

• Even cooling: Water heats up quicker than does oil, so it distributes the heat much faster and will allow the engine to operate at a better state than if you were just using oil alone.
• Cost: You change the oil in your car at a much greater rate than you do coolant. Even if you use year long protection oil (say the Mobil 1 stuff), it's only guaranteed for a year or 20k miles. Coolant typically lasts for 5 years or 150k miles (or something similar depending on what you use). If you want to increase the cooling capacity of oil, you'd also need to increase the quantity of it as well, which would in turn cost more money. Changing coolant every 5 years is a lot more cost effective then changing oil every 3-6 months or whenever your engine oil life meter tells you to.

There must be more reasons, but really, while oil provides cooling as one of its objectives (along with cleaning, friction reduction, and other reasons), lubrication to prevent metal on metal wear (IOW: extend the life of the engine) is probably its most important job. Making it do cooling as a primary job would shorten its lifespan, thus causing the owner the need to change it more often, causing more cost and waste. Regular coolant does a great job for what it's asked to do and what it's designed to do. We should let it do its job.

Answer 2

No, I do not believe that your engine as designed would possibly operate for very long without coolant in the water jackets. Even if ordinary engine oil was poured in there in place of the water, it would not work very well as others have explained.

I don't disagree with any of the answers listed to this point. However, I would say that once one begins to design such an oil cooled engine, they would soon find that it needed to have a separate system in order to be able to protect the engine from the contaminants introduced into oil due to cooling because that oil suffers greatly at the higher temperatures. Then they would discover that the oil used for cooling needs to have better heat transfer to work efficiently and to keep oil from getting scorched, so that with the separate systems they could make it more "water-like" in characteristics. Keep going with this (based almost completely on the other answers) and they will end up with an "oil" that is much like a 50-50 anti-freeze - which is what we currently use. This mixture uses other chemicals to enhance its abilities to withstand cold temperatures and reducing effects of corrosion while keeping most of the properties of the water intact and that is what the oil would have to do to overcome problems it faces.

So why do air-cooled engines exist? Probably because the weight of the material needed to provide the coolant jacket was greater than the cost of the materials for the 'fins' and had no significant advantage in cooling as long as there was oil circulating in sufficient enough quantities to help out. There is little doubt that air cooling has a difficult time to maintain uniform temperatures compared to liquid coolants, but the weight advantage of air using fins versus coolant using a coolant jacket is undeniable. It is made less if the coolant cooled engine uses the same types of materials as the air cooled engine, but that makes a large engine much more expensive. Somebody made the trade-off a long time ago and it has proven out over the years.

Who knows, keep thinking about it - someone might come up with a better cooling method for engines like these. Just look what they have done with electric motors over the years. Huge motors are now quite small with better torque response, etc. The same has happened to internal combustion engines though not quite as dramatically so far.

Answer 3

Paulster2 is correct, and I agree with his analysis. I'll add that engines (or, by extension, vehicles) can be only oil-cooled, but they have to be specifically designed for oil cooling. Not only does such a motor need greater oil capacity and greater capacity for oil > air heat transfer (large oil cooler with oil cooler fan, anyone?), the motor must be designed to tolerate the extreme heat differential between combustion chamber and block mass, both at startup and in hot weather under load. Thermodynamically, this is not trivial.

I ride a twin-cylinder BMW motorcycle that was designed and operates without liquid cooling. But on a 95˚ day, I'd rather be in the car.

Answer 4

As to street bikes: air cooling was long preferred for bikes because it saved weight and and was simpler than liquid cooling. Since the mid-80s, however, liquid-cooled bikes have become more common as their superior temperature regulation allows bike motors to be designed and built with closer tolerances, required for the motors to meet increasingly-stringent emissions requirements. (Many bikes now carry catalytic converters, too.) In addition, smaller clearances also yield quieter engines as the water jackets muffle the combustion and mechanical clatter; in these days of increasingly-stringent noise limits, this allows designers to hold down the engine noise and enable a bit more noise from the exhaust, which customers often prefer, and still stay within the limits.

Dirt bikes (i.e., those not registered for use on public streets) are subject to different regulations, with which I’m unfamiliar.

Answer 5

Water (with or without regard of coolant to lower freezing point and pressure cap to raise boiling point) has much greater heat capacity than oil, which in turn, limits the amount of reserve coolant capacity required to dissipate engine block heat build up. Of course, one could dissipate heat solely with oil but the amount of oil required (due to its lower heat capacity per volume unit) would be much, much greater than the equivalent heat dissipation capacity of water coolant; and, accordingly, the tank required to contain said coolant oil. Along with said addition of oil and oil storage tankage comes additional payload weight which, in turn, requires more heat generation to achieve the same torque and speed parameters as a water cooled engine. Bear in mind also, that in addition to dissipating engine block heat, an engine's cooling system also serves to cool engine oil.