Engines Exposed: Why Motor Oil Is So Important
In our piece about the various different types of combustion engine cycles, we discussed the efficiency that is possible from the various different ways that engines are built. However, the efficiency to which we were referring is the thermal efficiency of the engine, which describes the amount of energy that can be obtained from the heat of combustion.
We have covered some of the most efficient engines on the market, which are Atkinson cycle engines, and they are about 38% thermally efficient; the remaining heat is lost as exhaust or into the engine’s cooling system via the block. Even after 140 years of development, more than 60% of the total potential energy does not go toward powering the car. This seems egregious, and it is why so many manufacturers are working hard to improve thermal efficiency. However, engineers have proven their mettle by optimizing mechanical efficiency.
Mechanical efficiency refers to the amount of power that is extracted from a combustion when factors like frictional and pumping losses are taken into consideration. Effectively, it compares the amount of power that could be obtained from the combustion with the amount that is actually realized. Most modern engines are extremely efficient mechanically, with most having an efficiency rating of more than 90%.
Modern manufacturing techniques have made it possible to produce very high-quality engines at prices that are acceptable to the market, which has significantly improved mechanical efficiency by tightening tolerances in engines. However, even extremely sophisticated engines would suffer without that unsung hero of engine performance: oil.
Motor oil is more than just a reason to visit your mechanic on a regular basis (more on that later). It is the the protector of the engine. Maybe we should start a webcomic about Prince Pennzoil, who protects the engine in four different ways:
1. Lubricating moving parts
Lubrication is achieved by forming a minuscule barrier layer between moving parts. The lubrication, specifically between the piston rings and cylinder walls, helps to reduce friction, thereby reducing wear. This provides a longer life for the engine and better performance throughout its life.
2. Improving cylinder compression
The lubricating barrier mentioned above fills in any gaps between the piston ring and the cylinder wall. This improves the seal of the cylinder, which helps to improve the ability of the engine to compress the air-fuel mixture and to capture the maximum about of energy from the resulting combustion.
3. Cleaning and preserving
As oil is being produced, a variety of additives are introduced into the formula that help the oil to protect the engine from a variety of different conditions. These can include anti-frothing agents (bubbly liquids include a lot of air, which doesn’t lubricate as well), anti-corrosion agents (which protect against rusting), viscosity modifiers, detergents (which sequester carbon deposits and other insoluble materials in the oil), and pour point depressants (which allow the oil to flow at cold temperatures).
4. Cooling the engine
Oil isn’t as good as water at storing heat (lower specific heat), but it still has a significant conductive cooling effect on the engine via contact with internals and the oil galleries in the engine block. This is in addition to the heat reduction benefit of reducing friction between internal components. Many high-performance cars will feature oil coolers to help maximize this effect.
For most engines, oil is stored in the oil pan, which is mounted to the bottom of the engine block. This is referred to as a “wet sump” system. In a wet sump system, oil is moved through the engine either by direct contact or via an oil pump, which pumps oil through galleries in the block. As explained in a previous article, it is also possible for engines to have what is known as a “dry sump” system, where oil is stored in a reservoir that is disconnected from the block and then pumped into the engine.
There are two main families of motor oil in the automotive world: mineral (traditional) oil and synthetic oil. Mineral oil is refined from crude oil, and is a largely hydrocarbon-based product. Regardless of the source, the oil will have a viscosity rating. Most modern oil is a blend of two different hydrocarbons with different viscosity ratings, such as 10W-30. The heavier the molecular weight of the hydrocarbon chain, the greater the viscosity (or thickness) of the motor oil. The winter rating of the oil (hence the “W”) refers to the SAE viscosity rating of the oil when it is cold. For those of us in a cold climate, this is important to know — if you have ever heard an engine struggle to start on a frigid February morning, you’ll understand why you don’t want to add any extra burden. The other number indicates the SAE rating when the oil is at 210 degrees Fahrenheit.
While a blended mineral oil will perform far better than a single-grade mineral oil, which would be rated with a single number (e.g., 10), it is still limited by the chemical properties of the mineral oil from whence it came. Once again, science comes to the rescue: As early as 1877, scientists were experimenting with producing synthetic hydrocarbons. These would serve the dual purpose of providing an alternative source for oils, which is important given that petroleum is a non-sustainable resource, and an opportunity to engineer materials to meet the needs of the end user. Nearly 100 years later in 1972, the first commercial product passed the API testing to become available for consumers.
Since then, synthetic oils have grown in the public eye. This has been helped by an aggressive marketing campaign, especially by Mobil for its Mobil 1 synthetic product. Many high-performance cars are specifying synthetic oil for their regularly scheduled maintenance, which has increased its popularity. It is especially valuable in systems that are turbocharged as they help to lubricate the turbo, which can be spinning at more than 200,000 RPM.
Overall, oil today is a far cry from crude. It is intelligently designed to meet the needs of the modern engine. This has led to the interval between oil changes becoming longer over the past few years, with some manufacturers even suggesting 10,000 miles between oil changes. As with any maintenance, it is good advice to follow the manufacturer’s recommendation. One important note about maintenance intervals is that they are usually stated as “X years or XX,XXX miles” and it should be interpreted as whichever comes first. Once oil has been exposed to combustion gases, it starts to degrade; if you drive 1 mile home from the garage and park your car, the oil will still wear out with time. So do your car (and yourself) a favor and pay attention to your maintenance. Your car works hard for you, so giving it good oil to protect it is the least you can do.
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