How long could a car drive for, given unlimited fuel, unlimited fluids (coolant, oil…)? I’m thinking the first thing to give out after a long time of driving would be the tires, from general wear and tear, but lets assume that they’re magically changed on the go, when would the engine give out? Given unlimited inputs and the continued operations of all other systems (cooling, exhaust…) could it just drive forever?
Every part will eventually wear out. Unless you have a way to replace parts while still running then it can’t run forever. However, it’s conceivable to do that, the simplest way is a very tricky arrangement where you car is actually two cars, one piggy-backed on the other, and the piggy-backer can be transferred to the ground and then pick up the car that had been carrying it before. So one car is always driving while the other is being repaired on top of it. Even then you need good luck and a good crew to make sure the currently driving car doesn’t break down unexpectedly.
Your weak link is probably going to be the timing belt. Those are designed to be changed every 100,000 miles or so (more or less, depending on the vehicle). They are designed to be changed long before they break, so you’ll probably get to somewhere between 150,000 and 200,000 miles before it snaps. When that goes, the engine stops. If the engine has a timing chain instead of a timing belt, then it will last a lot longer. You’ll be able to go over a million miles on a timing chain, so that won’t be a problem.
Belts are lighter and quieter, which is why most cars use belts these days. Many trucks still use chains.
Your next weakest link will probably be either the water pump or the fuel pump. Those will probably go out somewhere between 200,000 and 500,000 miles.
Of course, a lot of this depends on how you drive it. If you are just tooling idly around a test track, then everything will last a lot longer. If you are simulating city type driving by constantly stomping on the gas and stomping on the brake, then things are going to wear out significantly faster. Keep the engine at a nice low 1500 rpm and it will last for hundreds of thousands of miles. Keep the engine close to redlined and its life will be measured in thousands of miles instead of hundreds of thousands of miles. Actually redline it, and its life may be measured in miles.
Also, the driver will have to get out to pee occasionally.
Although more seriously, cold start cycles are responsible for a goodly portion of engine wear and so if our hypothetical car is constantly running you should expect to see normal wear-out parts like timing chains and such lasting longer than normal. That’s how taxis can rack up huge amounts of punishing city mileage with minimal repairs; they’re just never allowed to cool down.
Given endless fuel plus coolant and oil, it would be easy enough to put an engine on a test stand and run it indefinitely. “Million mile” engines are becoming almost common in the trucking industry. Over at Detroit Diesel, they’re saying half of their engines are still running without needing a rebuild at 1.2 million miles.
Of course, that says nothing about tires, transmissions, suspension, etc. I suspect the most significant limitation on any of this will be pure passage of time - the environment is generally not kind to rubber parts.
To generalize this, you could have a vehicle with N independent drive systems. Each engine, fuel system, driveshaft, etc, is separate, driving separate sets of wheels that can be lowered to become the drive wheels, or raised when not in use.
With infinite supplies and labor for maintenance and a high N, you could keep such a system going for a very long time, since the chance of failure gets lower, the higher N is. I imagine the engineering of wheel placement becomes difficult for N > 4, and of course the systems aren’t totally independent, since if Engine #3 explodes, it might take out some important components in other drivetrains.
I’m going take the tack that you’re trying to design an engine that runs forever so there is no timing belt; maybe timing gears. Heavy equipment and truck engines are pretty much designed to run forever. You COULD design an engine with no seals to wear out and no pressurized fluids - look a Model T.
For a “forever” design I’ll say that the failure mode will something that fails from cyclic load like rod bearings, cam lobes, valve springs. (Plain bearings with oil film and a constant load CAN be designed for infinite life.) A second predictable failure mode would be piston rings wearing to where there is too little compression. Finally, unless you assume a perfect operating environment many failures can be caused by external influences like grit getting past the air filter, combustion products polluting and adding grit to the oil, environmental temperature changes stressing gasketed joints.
I hesitate to question answers provided by someone whose handle is engineer_comp_geek but I did study auto mechanics once upon a time.
In far less than 100,000 miles I think the following components would wear out:
• fuel pump, assuming this isn’t being regarded as “not part of the engine” (and it should be, I think, insofar as the mechanical ones, at least, are driven directly by an engine shaft). I may have to start buying mine from whever engineer_comp_geek gets them, but on the cars I’ve owned and maintained they die 50K-70K miles.
• water pump, same disclaimer about assumptions — driven by a fan belt powered directly by the engine crankshaft; again, my mileage apparently varies
• spark plugs — they pit and corrode and after awhile won’t fire; they aren’t a consumable or a liquid like oil or gas, so why wouldn’t they qualify? They’re screwed right into the intake heads.
• radiator hoses — I’m possibly reaching a bit on this one to consider it “part of the engine” but it still isn’t a consumable or a liquid like oil or gas. The rubber wears out from the heat and pressure, they split, leak, and your engine isn’t going much farther without coolant.
• oil filter — technically not a liquid or consumable, although it tends to be changed when the oil is changed; if it counts, here’s another weak point in the system; left unchanged, they clog up with carbon particles and varnish and impede oil flow to the engine.
• fan belts —would you count them? the ones driving the alternator and the water pump are crucial to the engine’s continued operation
I’d suggest cold-starts are responsible for the majority of engine wear.
If the oil, coolant and tires are magically renewed on the go, then what’s left to fail in service? A few thoughts:
-If the oil filter isn’t changed now and then, it will clog up with wear debris, creating enough backpressure to open the bypass valve. Unfiltered oil will start circulating through the engine. I don’t know how long it takes for this to happen, but I’d bet it’s less than 100K miles.
-The intake air filter will get progressively clogged, making it harder for the engine to breathe. The driver will compensate by opening the throttle a little more, but at some point the throttle will be wide-open, and the engine will barely suck enough air through the filter to make the desired power. When things get really bad, the manifold vacuum will collapse the intake filter. If you’re lucky, you won’t have caked-on dust falling off of the filter and into the unprotected intake, but even if you don’t, the engine will start breathing unfiltered air, which will accelerate the wear process. The pacing of all of these events of course depends on where the vehicle is being driven. Headed out into a perpetual haboob? I’ll be impressed if your filter doesn’t implode before 100K miles.
-Spark plug electrodes don’t last forever. Each ignition event removes a tiny piece of material from the electrodes, causing the spark gap to increase in length over time. Modern spark plugs are supposed to be changed perhaps every 100K miles, which provides a generous safety margin. If you start going well past this, the increased gap results in increased voltage, which stresses the ignition coil. If things get bad enough, the coil can arc to ground through its insulation, damaging it and making the spark for that cylinder unreliable or nonexistent. How many miles can you go past this before you start having problems? That will vary depending on the engine and on how it’s driven. I’d be impressed if problems didn’t start surfacing after 150K miles.
-Some users at TireRack are reporting well over 100,000 miles on a single set of tires. My guess is that highway driving will be kinder to tires than city driving; you won’t have a constant barrage of hard acceleration and braking, just a modest tractive force to maintain crusing speed. With that, maybe you could get 150K from a set of really hard tires?
Took a while to write this; some of it got ninja’d by Ahunter3…
You could design out most of these issues. All the diesels I have seen use mechanical fuel lift pumps, and there are no spark plugs to wear out. The injectors will of course eventually drop off in performance, but it takes a lot for them to quit working entirely. You could have an oil reject system that keeps the filter clean, but that might be exploiting the surplus oil premise. The water and fan could be driven by more reliable means than a belt, although I don’t know if I have seen any actually implemented.
A big question is how fast the clutch wears out, and if it might be preferable to use an auto with a torque converter.
If you build it logically, so that every part is just as strong as every other part, then there is no possibility for any part to break down before any other part. The vehicle as a whole will eventually wear out and collapse in a heap; but this strategy of vehicle construction should easily give you a vehicle that runs for a hundred years. This was allegedly a known strategy as far back as the mid-18th century.
Most of these things today area easily 100,000+ mile parts on an ordinary production vehicle. The alternator in my truck (a 5.7 liter Hemi gasser) was showing signs of wear at 115,000 miles, so I swapped it out on my schedule rather than waiting for a complete failure at an inconvenient time. The serpentine belt was still in fine shape, but I chose to replace it and the belt idlers/tensioner, again, to prevent a breakdown on some rainy night far from home. Same engine still has the original radiator hoses, but I’ll probably replace them this summer just for peace of mind. Oh, I’m still running the original water pump and fuel pump. Not sure of the water pump, but the fuel pump and fuel filter are both considered “forever” parts unless you burn out the pump by running it dry.
Two people flew a Cessna 172 for over 64 days straight and over 150,000 miles. They refueled by flying close to the ground and pumping fuel up from a truck. They arranged the oil system so that the oil and filter could be replaced in the air. They swapped pilot duty every four hours for sleep and other biological needs. They stopped only after carbon buildup in the cylinders caused so much power loss that they couldn’t climb easily after refueling.
The Cessna story is awesome! There is probably some kind of equivalent record of endurance set in a car.
Depends on the mission. If it’s driving on a test track you only use the clutch about 5 times in its life: starting from a standstill, shift to 2nd, shift to 3rd, shift to 4th, shift to 5th, drive 1 million miles.
In that scenario I’d far rather have a manual trans. swap out the lube every 100K miles or so and it’ll last multiple millions of miles.
I haven’t seen anything similar–there are some endurance runs but nothing that doesn’t involve pit stops.
I wonder if a Tesla can be recharged while moving? That seems like the way to do it: find a long, circular track, set the Tesla to auto-driving, and every so often have a guy in a truck drive alongside and charge it up. No oil changes or the like needed, and the drivetrain should easily go hundreds of thousands of miles.
How odd … The FAA lists the registration number (N9217B) as a 1958 Cessna 175 … current owner one Jimmy L Womack of Robert Lee, Texas.
The plane is hanging in a terminal at the Las Vegas airport: https://upload.wikimedia.org/wikipedia/commons/8/8d/Timm-Cook_Cessna_172_LV_NV.jpg
Looks like the number is really N9172B.
Thanx … I need to get eyeglasses I guess …