There are around 1.42 billion cars on the planet, but not all of them are all-that reliable. Some are old beaters that can barely start-up and others can run for a million miles. But what makes all of these cars so different? Why can one car barely survive its intended lifecycle and others seem to outlast its owners?
The reality of what makes a car reliable, or rather, what makes an engine reliable, all comes down to how the engineers that designed it handled and mitigated the stresses of it operating. Engines handle some insane environments. On one end, they can survive in extreme weather conditions, but what happens inside is an even harsher climate for engineering. From fiery explosions every few seconds to hot and cold cycles with oil gushing about, getting a machine with thousands of parts to handle this environment is no easy feat.
What causes engines to be unreliable?
In order to understand why certain engines are unreliable, we first need to define unreliability. Particularly, motors cease to function when they are considered blown, when damage is done to some part of the main engine block that requires either a new engine or the existing engine to be rebuilt. Motors that last 1 million miles can survive that long with regular maintenance, making them ultra-reliable. Unreliable motors will often only last a few hundred thousand miles before blowing.
Engines generally blow for one main reason: overheating. When an engine gets hot, all of the components of the engine warp and expand. Engineers design for these expansions within a certain range but when an engine reaches temperatures outside of its intended range, seals and gaskets can blow — or worse — cylinders and other components can catastrophically fail under the stress.
How engines combat heat
Engineers combat overheating in engines through cooling systems and oiling systems. Cooling takes the heat of the engine away from the cylinders and dissipates it out from the radiator via the engine coolant that travels in channels throughout the engine. Cooling systems are a heat-management device. Oiling systems, on the other hand, are heat prevention devices. By keeping a car oiled and lubricated properly, friction can be impeded in the engine, therefore keeping it cooler and within operating temperature.
Looking back to cooling systems, one way that an engine can become unreliable is if the coolant channels don't efficiently or evenly cool the entire engine. If uneven cooling in an engine occurs, different parts can be different sizes due to thermal expansion, creating a potential failure point. In some unreliable engines, the coolant channels that run throughout the engine can be too small, meaning that not enough heat is absorbed and removed from the block by the coolant. This can compound when a vehicle with already poor cooling moves at slow speeds. Because cooling systems rely on radiators to dissipate the heat through conduction with the air around them when a car moves more slowly, less air passes over the radiator and thus less heat is radiated out.
The capacity of engines
All this talk about coolant and oil brings us to one of the first main design traits that can make an engine ultra-reliable: high capacity. Engines that have high capacities for oil and coolant generally do a better job of handling the thermal stresses in an engine. Think of this as having more ammunition to fight an enemy. When engines have more room for coolant and oil, they have more firepower to fight back against the engines, well, firepower.
It's important to keep this discussion in general terms though. Small engines can be made reliable through other factors and adjustments. Big engines aren't naturally more reliable just because they're big, and conversely, small engines aren't naturally unreliable because they're small.
Diesel vs. gasoline
The next part of reliability to discuss is diesel versus gasoline. Diesel engines are widely regarded as reliable powerhouses that are good for towing and long-distance travel. But why? The reason for this has to do with lubrication. Diesel fuel is more lubricative than petrol, meaning that as the fuel is injected into the cylinders, it actually helps the oil lubricate the engine and keep friction to a minimum. Gasoline, on the other hand, usually has detergents in the fuel, causing the fuel to clean the engine of oil and other grime as it runs throughout the cylinder.
But again, diesel versus gasoline oiling differences aren't a solve-all, rather, the differences make up one small notch on an engine's journey to ultra-reliability.
Heads and blocks, aluminum or iron?
Looking next to engine design, nearly all engines have a head and a block, the two main pieces of the engine. Between these two pieces is a head gasket, a very common point of failure for engines. The head gasket seals up the connection between the head and the block, protecting the coolant and oil channels from spilling into the cylinders. When a head gasket blows, coolant and oil, or even fuel can spill out into places it isn't supposed to be.
The key takeaway from this design is that the head and the block are two different pieces of metal, meaning that they can have different rates of thermal expansion. Engineers can generally choose to design a block or head from aluminum or cast iron. Both have their pros and cons and some engines mix and match the metals. Cast iron is stronger and cheaper, but it's also heavy and distributes heat poorly. Aluminum on the other hand is light and has very good heat distribution, it's also more expensive and tends to expand a lot under heat.
Some of the most reliable engines are designed with a strong cast-iron block and an aluminum head. This design allows the main structure of the engine to be strong while the head of the engine can dissipate all of the heat. But this design has to be handled properly to work. Some of the least reliable engines in the world also have aluminum heads and iron blocks, because they improperly deal with the thermal expansion problems this design presents.
Another factor that influences the reliability of engines goes beyond thermal forces and rather deals with kinetic forces from the movement of the pistons. In engines, there are primary forces, which are created from the in and out the movement of the piston in a cylinder. There are also secondary forces, which are the forces from the shaking or moving around side-to-side of the pistons in the cylinder.
The way these forces are dealt with primarily comes down to engine layout. Inline four-cylinder engines cancel out primary forces by having pairs of pistons on the opposing sides move up and down synchronously. However inline-four engines suffer from an imbalance of secondary forces. Straight 6 engines on the other hand are capable of balancing out both their primary and secondary forces through proper piston timing, making this design one of the more reliable statistically.
All of this talk about the specific reliability traits doesn't tell the entire story though. One of the most famously-reliable engines to have ever existed is the Toyota 2UZ. This engine is a cross-planed V8 which requires added counterweights to balance out its forces. It also has a relatively low oil capacity compared to other engines of similar sizes. Finally, the engine has an iron block and an aluminum head. On paper, this engine seems like it wouldn't be as reliable as a full iron block large oil-capacity engine. So why does it work? Because owners of vehicles with this engine tend to drive longer periods of time at once. They put their cars through fewer engine cycles.
This final trait that can make an engine reliable has less to do with the engine design and more to do with how it's used. An engine cycle is defined as the engine going from cool to hot to cool again. Some of the longest-lasting engines are those found in vehicles that are used for long-distance driving. This is because driving engines over longer distances in one go results in fewer engine cycles than a car that takes many stops to travel the same distance.
It's this facet, engine cycles, that is really a better indicator of an engine's "mileage" or wear and tear. For example, cars that have driven a million miles tend to be ones that were driven long distances over their life. When used in this fashion, a car with 1 million miles might have the same number of engine cycles as one with just over 100K.
When a car goes through an engine cycle, the engine parts expand, rub against each other and constantly change states. This rubbing can create failure points in the engine. By driving long distances at once, the engine stays in a constant hot state, avoiding the constant rubbing and grinding that thermal engine cycles create.
So, what makes certain engines reliable? It comes down to clever engineering to allow an engine to properly handle thermal stresses through any design characteristic possible. Some engineers develop engine designs hoping to be clever and find the next methodology that may fail once put into practice. Engine design is a science that requires the careful balancing of a multitude of factors.