Generic question on longevity of modern automotive engines at high load 2

[LMF5000]

This is a "what-if" question - asked mainly out of curiosity, not because I need to implement any of the scenarios mentioned.

Consider a modern European hatchback designed for fuel efficiency and low price - so low displacement turbocharged engine is the order of the day. Say, 1.0-1.5 liter 4-cylinder petrol engine making 100-200 bhp with turbo. And in a car that weighs about 1200kg. Top speed of around 200 km/h.

Question 1 - imagine the car is driven for several hours daily on the Autobahn, at its top speed. How long would it be expected to last? Which component would fail first? Are cars of this size even designed with enough natural cooling capacity to withstand several hours of full throttle operation?

Question 2 - imagine the same automotive engine, but this time driving a stationary load (like a generator or water pump), or used as a boat inboard engine, or an aircraft engine. In each case assume the engine has sufficient cooling (via cold water supply or oversize radiator), no unnatural axial loading on crankshaft (i.e. propeller thrust loads borne by thrust bearing not directly loaded on crankshaft), and engine spends all its time at 80-100% of rated power. How long will it last this time, and which component fails first?

Reason I'm asking is because modern automotive engines strike me as taking advantage of the fact that full power is used only briefly in a car's typical operation, so they have very impressive specific power figures (over 100hp/liter) - but I can find no data on how durable they are when producing high power for extended periods of time. I'm hoping some automotive engineers can shed light on this question.

P.S. This being my first post, I should probably introduce myself and provide some background. I'm from Malta, have a B.Eng in mechanical engineering and a masters in materials engineering. My current job is package development for a semiconductor assembly plant, main focus being R&D of novel MEMS device assembly processes.

 

[tbuelna]

Mike, tbuelna - do you have any idea of the longevity of an automotive engine converted to run an experimental aircraft, in terms of hours between overhauls?

LMF5000-

The duty cycle of a recip piston aircraft engine is far different than a typical auto engine. The auto engine operates most of its life at light load and low speed operation. The recip piston aircraft engine has a duty cycle (ground-air-ground cycle) that involves a short period of high speed, high load operation during take-off and climb, followed by extended periods of operation at 60%-70% speed and full load. So it seems likely a recip piston auto engine used in an aircraft would have significantly reduced MTBR due to mechanical/thermal fatigue issues in many of its components.

 

[berkshire]

LMF5000,
Your friends price for a 50hp Rotax engine sounds off by a factor of 10. The highest price I can find is about 7000 euros for a 503, or 582 ,or similar with a gearbox and electric start .
B.E.

You are judged not by what you know, but by what you can do.

 

[LMF5000]

Berkshire - you're correct, that is a little high. It's a Rotax 582. Using the configurator here I recreated their setup for 10,900 CAD (€7500). Add €2000 for a carbon fiber prop, and shipping costs to Malta. Not sure what else is included in the €30k figure. I will ask next time I see him

 

[berkshire]

LMF5000,
The Subaru engines they are using in the RV 9's are 3 litre 6 cylinder boxer engines generating about 180 to 200 horsepower through a reduction gearbox using an MT electric constant speed 3 blade prop.
B.E.

You are judged not by what you know, but by what you can do.

 

[LMF5000]

Hi Berkshire,

Thanks for getting back to me. Interesting how they both use horizontally opposed engines - as are certified engines. Perhaps they have the advantage of being lighter because of less balance weight?

Anywhay, very interesting that they use an electric constant speed prop. I didn't know they existed until you mentioned it. Only ever saw planes with fixed pitch and hydraulic CS props.

 

[dicer]

You question the requirement of dual ignition systems in aircraft engines. And yes the rules are based on the old days.
When most all ignition systems used breaker points. They always were the weak point in the system, also the high tension components could be bothersome as well, all items that make redundancy a good thing. It would have been much more difficult to use 2 spark plugs per cylinder using one distributor or one magneto than it is with 2 or more.
2 spark plugs on an aircraft engine should be a necessity especially in the old days, with the higher TEL concentrations used then. Spark plug fouling was pretty common. And saying nothing about the efficiency increase with the extra ignition source.

As far as I know any durability testing on automotive engines using 100% plus of rated power are all done using precisely controlled conditions on a dynamometer. I've also questioned if the cooling system and exhaust system in the average car would withstand full power output for extended time. I think the average car uses maybe 20% of max hp in normal driving.

 

[GregLocock]

Rule of thumb 30 years ago at two different manufacturers was that the prototype of a new design of car engine had to survive for 100 hours at full power on the dyno before the base design could be signed off, and at that point it was released to the mainstream development team. As to what the actual life of a /production/ engine at full power would be on a dyno, I don't know. As you have pointed out this is a serious bit of design overkill, real drivers in most countries spend remarkably little time at full power, for the most part (yes, I have the data). If they have autos they actually spend very little time above 60% red line. Incidentally the running-in event that killed most engines on a dyno is full revs with no or light load, reputedly.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[LMF5000]

"Incidentally the running-in event that killed most engines on a dyno is full revs with no or light load, reputedly."

I've heard it said before that revving an engine with no load is dangerous. The (non-technical) sources I read allege that the harmonics that would otherwise be dampened by the load are instead allowed to run free and stress the engine. Personally I've never exceeded 4000 revs in neutral. I hear the modern VWs also have a drastically reduced rev limit in neutral or with the clutch pressed - though whether this is to prevent damage, or actually meant to be a primitive form of launch control I don't know.

 

[Lou Scannon]

I remember reading in a contemporary Car and Driver article on the all new 1984 Corvette, that the driveline and specifically the hydraulic clutch were engineered to withstand "sidestepping" the clutch pedal...

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[NormPeterson]

I wonder what that really meant. Conceivably the hydraulics could be arranged to permit a maximum flow rate and consequently mitigate the harshness of a sidestepped clutch engagement. Beneficial for most everything but clutch disc wear, for which warranty replacement could be denied based on "abuse".


Norm

 

[imcjoek]

Frankly the 84' Vette didn't have enough power to abuse much of anything...

 

[crerus75]

Some kind of flow restrictor is used in the hydraulic circuit of at least some automotive clutches. When I was an auto tech the guy in the next bay wound up putting an unsprung-hub, ceramic-metallic-puck racing clutch in an AWD Mitsubishi Eclipse as per the customer's request. Part of the job included removing some sort of restrictor in the hydraulic circuit. The clutch manufacturer (I want to say it was Clutch Masters, but it's been a long time. I remember the pressure plate was painted white) advised that sidestepping the clutch without the restrictor in place would probably result in the generation of spare parts in a hurry, and that it was only recommended for "race applications". I don't know how it was to drive, but I imagine that damned clutch snapped shut like a bear trap every time the owner pulled away from a stoplight.

 

[Lou Scannon]

As I recall, the 84 Corvette sidestep manouever was described in the article as intended to give a satisfyingly hard launch and/or wheelspin.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[dgallup]

At 240 HP it was considerably more powerful than it's predecessor (200 HP) and pretty much everything else on the American market save some exotics costing 3 times as much. It was the first multipoint EFI from Chevy. We had two in out test fleet and they were way faster than the BMWs & Jags, even the V12 XJS (which was rated for more power but must have weighed at least 1000 more pounds and was crippled by a 3 speed automatic).

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[crerus75]

dgallup, IIRC the 240 hp rating was for the later TPI engines. Early cars had less, and 84 was Crossfire-injected (two TBI units on a crossram manifold, RPO code L83). I think this gave way to an iron-headed TPI engine (RPO L98) in 85 and half of 86, and an aluminum-headed TPI from late 86 to 91. The early cars also had a Super T10 manual with a Doug Nash overdrive that could be engaged in the top 3 gears. This gave it the name 4+3. The transmissions were good but the overdrive units were trouble prone.

 

[LMF5000]

I can't say I really see the point of the sidestep manoever. My first car had 80bhp and it could do burnouts just fine simply by releasing the clutch quickly (by foot) at 2000rpm and flooring it. I don't really see the point of lengthy burnouts themselves to be honest, except for purpose-built drag cars that need to warm up the slicks before a run. And for showing off, obviously

 

[crerus75]

It is said that a sidestep engages the clutch more quickly than conventionally releasing the clutch. Put another way, the clutch can engage faster than the driver's foot can come up, so sidestepping prevents the clutch pedal from "riding" the sole of the driver's foot on the way up. I've never timed it, and I would suspect that the time difference is not very big, but sidestepping does eliminate the chance of getting your foot hung up when your snow boots and the pedal bracket of your college roommate's 1986 Z-28 try to inhabit the same space at the same time.

 

[dgallup]

crerus75 - Those Cross Fire dual throttle body systems were awful, I thought they were only on C3 Vetts and Camaro/Trans Am but you are right, the first year C4 had it too. Maybe our 2 C4 Vettes were prototypes or maybe they were actually 85's, definitely had port injection. That was back when Chevy had massive problems with fuel injector plugging on the small block V8's. We did a lot of testing and had a driving loop that could plug the OE injectors in about 2 weeks with high olefin fuel. Our injectors were the OES fix but you had to take your Vette back 3 times with injector problems before they would replace them. The first 2 times they just cleaned them which was totally ineffective.



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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[NormPeterson]

Put another way, the clutch can engage faster than the driver's foot can come up, so sidestepping prevents the clutch pedal from "riding" the sole of the driver's foot on the way up. I've never timed it, and I would suspect that the time difference is not very big . . .

At the drag strip, the time difference to full clutch engagement is likely less important than the lower likelihood of having the clutch go into some sort of full slip condition as a result of slower engagement.

More abusive than I care to subject any of my cars to.


Norm

 

[SomptingGuy]

I always thought side-stepping the clutch was to simulate an "idiot start" - one where a driver mistakenly slips off the pedal instead of releasing it progressively.

- Steve