Starting an engine puts tremendous straining on the moving parts--no oil pressure to lubricate. So save your gas, and buy a new engine/new car prematurely. How green is that?

The Prius engine stops and starts often and there have been no durability problems reported. The efficience advantage of engine shutoff in advanced internal combustion powered vehicles is well established.

"advanced internal combustion"

Advanced internal combustion engines??? What pray tell has changed in the lubrication of an IC engine since its been invented? There's an oil pump, an oil pan, oil filter, possibly an oil cooler. The only real advances in engine technology have been in the area of fuel delivery systems (including the EGR system) all the rest is about as basic and old school as you can get.

The point that the original poster made is perfectly valid, stopping and starting your car is a traumatic event due to the lack of oil on the moving parts. In a Prius that may not matter (I've not seen the experimental data to back that claim) if the stop and start times are small in duration - say before the oil has a chance to drip back down into the pan - but if you're gonna be stopped for longer than 10 seconds a substantial fraction of the oil has made it back down to the oil pan.

in the last 20 years or so the other major improvement in engines after fuel injection is the use of cnc machine tools that allow engines to be built with much tighter tolerances.this means that bearings and pistons fit there respective bores much better , preventing slop in the components.the oil films don't have to be as thick as in the old days.also with carburators there would be alot more raw gas getting into the combustion chambers at startup which would dilute the oil in the crankcase.

Yes, but while close tolerances remove the need for a large amount of lubrication, they increase the need for a constant flow of it. Loose parts aren't touching as closely as tight ones, and hence have somewhat less friction. Bottom line, if you're starting and stopping your care 20 or 30 times per hour, you're not only probably holding up traffic, but you are also increasing wear and tear on the engine.

part of the engine in stop and go traffic, and the gasoline engine is just there for highway speeds, and when the battery is low. True or not? If true, then you are starting an electric motor frequently--something you experience with your house heating and cooling system. Electrics have a lot less need for lubrication.

There is still some oil in the bearings and on the cylinder walls when the engine is shut off. Modern engines have a check valve (in some models in the oil filter) to prevent the oil from draining through the oil pump back into the crankcase. When the engine is restarted the oil pressure is re-established before the torque would increase to the point of causing wear.

There is a disproportionate amount of wear that occurs after a cold start, but we are talking about a hot start.

One advanced ICE powered, non-hybrid passenger car which employed automatic engine stop/start was the VW Lupo 3L. <link>

Lupo 3L

The Lupo 3L was a special-edition made with the intent of being the world's first car in series production consuming as little as 3 litres of fuel per 100 kilometres (78 miles per US gallon or 94 miles per Imperial gallon). To achieve this the 3L was significantly changed from the standard Lupo to include:

• 1.2 litre 3-cylinder diesel engine with turbocharger and direct injection (61 hp, 140 Nm)
• Use of light-weight aluminum and magnesium alloys for doors, hood, rear-hatch, seat frames, engine block, wheels, suspension system etc. to achieve a weight of only 830 kg (1830 lb)
• Tiptronic gearbox
• Engine start/stop automatic to avoid long idling periods
• Low rolling resistance tires
• battery location moved to boot for better weight distribution

During the period of series production of the Lupo 3L, Volkswagen also presented the 1L Concept, a prototype made with the objective of proving the capability of producing a roadworthy vehicle consuming only 1 litre of fuel per 100 kilometres (235 miles per US gallon).

STD-I think that's the valid point about hybrids. You don't have the same "cold start" issue that you have with straight gas engines because of the electric motor kicking in to start out. I would be interested to see more about what goes on as the gas engine cuts in and out with regards to lubrication.

Stop, you assume the oil between bearing surfaces instantly disappears when the oil pump stops.It takes time to do so. Much longer than the few moments that the engine sits idle.

The most wear would be to the starter motor, and it would be minimal if the engine starts instantly.

Stop, you assume the oil between bearing surfaces instantly disappears when the oil pump stops.

The oil doesn't suddenly disappear, but the oil pressure does. When an engine is running with proper oil pressure, the bearing surfaces shouldn't touch at all, the thin film of pressurized oil completely separates them. The presence of unpressurized oil will reduce metal-on-metal friction, but will not fully separate the metal surfaces like pressurized oil does. Wear will increase.

Those tight tolerances increase, not decrease, the need for a consistent flow of oil to the proper locations. Some other posters have made valid rejoiners, the answer to this question really depends on how long the engine is off before it is restarted. I myself, based on my own experiences in the engineering field, would consider 10 seconds overlong to risk it in a conventional engine. For a car designed to shut down and turn on for short periods of time - perhaps that's not the case.

The bottom line for me - is it worth it to risk screwing up the rings based on advice given in the Green Lantern? No, if I'm gonna do that I'd need a letter from Ford Motor Company first. This is the kind of advice that Slate should not be giving out.

great, fun car, but I had 2, count 'em, 2 timing chains replaced in under 60K miles. I changed the oil regularly, etc. but the chains still went out. After the second chain went out (fortunately, under a warranty extension) I asked the mechanic what the heck was going on. He explained that the timing chain was not well lubricated because it was the last step in the oil line, so it had the least oil pressure, ergo, more wear. As you know, timing chains should last a lot longer than 30K miles--the second chain was replaced under an warranty extension, up from 36k to 60k. I interpreted that extension as an admission by Nissan that they had a problem. When the car started making the same characteristic ticking sound, I sold it and go a new car. The job to fix the timing chain must have been big, since they kept the car for 2 days.

You should be able to get 200K out of a modern engine, but with low oil pressure, I would think not. I don't know if my estimates are right, but it makes no senses to save a couple of hundred bucks a year on gas, spend 3 grand every 3 years doing a major engine overhaul, Not worth it.

Prius engines (like all hybrids) are designed for this constant start-stop behavior and include many design features to allow this without premature wear... For instance, the engine is pre-spun without firing (no fuel or spark) to allow oil pressure to build.