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| Early direct injected Mercedes-Benz diesel, circa 1963 |
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.
He then turns to the reliability question. Why do some motors last a million miles (with maintenance) and other blow apart at 200,000?
The primary reason engines blow is 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.
To deal with all this heat engineers developed cooling and lubricating systems. Coolant conveys the heat away from the engine. Lubricants reduce the heat generated by friction and also serve to mitigate heat.
English breaks down the various way design failures in these cooling systems can lead to premature death for an engine at certain failure points. Cooling systems are also dependent on radiators to dissipate the heat, which is another variable in the reliability equation. It's more than just having oil and coolant though. Oil and coolant capacity is also a variable. This, he states, is one of the first design features that can make an engine "ultra-reliable."
Interestingly enough, there are ways that small engines can be designed to be more reliable. And just because an engine is big, he states, it doesn't mean it will live longer.
Diesel vs. gasoline
The heart of this story for us comes at this point after all the other groundwork has been laid. Why are diesel engines regarded as more reliable, especially for long distance travel and towing? The answer, he states, has to do with lubrication. The fuel itself is more lubricative than gasoline.
What he is referring to is what happens when the fuel is injected into the cylinders. Diesel fuel helps lubricate the enhine and minimizes friction. By way of contrast, gasoline usually has detergents that clean the interior of oil and other grime inside the cylinder.
Additional considerations
The article is essentially about reliability and all the decisions engineers must consider when creating a new engine design. For example, what material to use for the head and the block, as well as the head gasket, which is a common failure point.
"Another factor that influences the reliability of engines goes beyond thermal forces and rather deals with kinetic forces from the movement of the pistons. The way these forces are dealt with primarily comes down to engine layout," he explains.
The various kind of engine designs are discussed next followed by a discussion of engine cycles as it goes from cool to hot and back to cool, and how this has a bearing on an engine's durability.
In short, this is an exceedingly thorough discussion that is worth reading in whole. We've only touched on some the highlights here. Be sure to check it out. If you do not have tine to read it now bookmark it for a good weekend diversion.
What Makes an Engine Reliable
https://interestingengineering.com/what-makes-an-engine-reliable
See also: Why Do Diesel Engines Make More Torque
https://theblueflameblogger.blogspot.com/2021/01/why-do-diesel-engines-make-more-torque.html
Diesel engines do last longer than petrol ones. Diesel is a light oil and when burned and used as fuel by the vehicle it lubricates the parts of the engine. This prolongs the life of the engine. Petrol is a detergent and washes away oil from the components of the engine, therefore, wearing it out quicker.
ReplyDeleteAnother reason diesel engines last longer than gas engines: diesel fuel is a type of distillate fuel, that is essentially produced from crude oil, which gives diesel engines slower cylinder wear than gasoline engines. This gives diesel fuel lubrication properties that extend the life of your engine. On the contrary, gasoline is primarily made of aromatic hydrocarbons that act similar to solvents, that are harsh and corrosive with no lubricity and therefore wears the components of your engine faster. Diesel engines have lower Exhaust Gas Temperatures (EGT’s) which also helps their longevity. Although Diesel Fuel has more BTU’s, 139,000 versus 115,000 BTU’s for gasoline, the laws of thermodynamics indicate that the expansion rate of higher compression ratio diesel engines actually cool the exhaust gases faster. Coupled with the lower auto-ignition temperature of about 410°F for diesel fuel compared to the 495°F of gasoline, the initial flame front is cooler. Diesel engines also run at a much leaner air to fuel ratio which can be anywhere from 25:1– 70:1 as opposed to 12:1 – 16:1 for gasoline, so the extra air helps cool the EGT’s. Gasoline burns much quicker than diesel fuel. Less shock to the rotating assembly occurs because of the slower laminar speed of the flame during combustion which aids in the durability of diesel engines.
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