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.

 

[Lou Scannon]

Since this thread has derailed into a discussion about abusing engines and drivelines, I'll continue the derailment by sharing a fantasy of mine that was prompted by the Home Depot pickup truck rental offer, 75 minutes for a flat rate around $20. In my fantasy, I would go to the Home Depot with floorjack, a pair of jackstands, a brick, a lawnchair, and maybe some popcorn. I would rent the truck, jack up the rear axle and support it with the jackstands, start the engine, put the transmission in drive, and place the brick on the gas pedal. Then I would sit in the lawnchair enjoying my popcorn for the remainder of the 75 minutes, with the engine bouncing against the rev limiter, and the odometer racking up miles at 100mph+

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[GregLocock]

Oh, you mean the old night shift high speed durability test!

Cheers

Greg Locock


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

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

 

[LMF5000]

hemi - that's a good idea. The ensuing conversation (when they discover you added 125 miles in 75 minutes) would be hilarious...

Course it's based on the speedometer being connected to the driven wheels not the other axle

 

[dgallup]

What makes you think they don't have a governor on that rental that limits it to 70 mph?

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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.

 

[berkshire]

On a couple of rental trucks I have used, the governor was set to 55mph The truck would accelerate like a scalded cat to 50 then struggle for the last 5mph.
B.E.

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

 

[LMF5000]

Since we seem to have derailed this thread into a general discussion on cars (and I'm the original poster), I'd like to take this opportunity to ask another (completely irrelevant) question.

I just skidded into a kerb with my car. Right front tyre hit pavement while steering was at full left lock to try and counter the skid. Forward speed at impact was well under 30km/h.

Car is now pulling slightly to the right. Needs 15 degrees of left steering input to go straight. Steering wheel returns to perfectly horizontal position when released with car moving.

Hit tyre is holding pressure fine (and is within 0.5psi of left one). Rim is scratched but not bent. Control rod looks visually straight, same as one on other side (didn't check wishbones). Acceleration and braking don't affect the pull. Wheel balance is subjectively fine (no vibration) at up to 60km/h (didn't risk going faster).

Can you experienced gentlemen please provide some suggestions? Would a simple wheel alignment likely work?

 

[berkshire]

A "simple wheel alignment" will point you in the direction of the problem, you most likely bent something.
B.E.

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

 

[Lou Scannon]

On a couple of rental trucks I have used, the governor was set to 55mph The truck would accelerate like a scalded cat to 50 then struggle for the last 5mph.

Well, in that case, I would include in my kit a device for calibrating the computer settings (no rocket science these days), and move the road speed governor out of the way!

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[LMF5000]

will point you in the direction of the problem, you most likely bent something

How unfortunate. So a wheel alignment shop won't be able to bend things to cancel out the problem?

 

[berkshire]

I am afraid that at this point you just have to go find out.
B.E.

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

 

[NormPeterson]

So a wheel alignment shop won't be able to bend things to cancel out the problem?

Sounds like you bent the steering arm on the right side knuckle just a little (BTDT, left side).

If that is in fact the case, it'll be easier and far better to get it replaced rather than trust straightening it out, and I doubt that a repair/alignment shop would accept liability for straightening it anyway.


Norm

 

[LMF5000]

Took it to a wheel shop for inspection. Bottom right wishbone is bent inwards - the paint has chipped off in the center of the wishbone where the kink is worst. Steering link seems OK but steering is very slightly harder to turn than usual. Car has all-electric power steering system, no hydraulics.

The right wheel is also about 1cm closer to the back of the car than the left wheel is (i.e. can only fit 4 fingers between tyre and mudguard on RHS compared to 5 fingers space on LHS).

This car has a basic MacPherson strut suspension - single shock absorber on top, control arm (wishbone) on bottom. Like so:

I will be un-bending or replacing the wishbone and doing an alignment afterwards. Could something else be bent as well, besides the wishbone, to be causing the lateral displacement of the wheel towards the back?

 

[Tmoose]

"Could something else be bent as well?"

After the shop did their inspection did they mention suspicions of damage beyond the control arm?
How sure are you (how sure >>can<< you be ?) that the wheel was not displaced prior to whacking the curb?

Was the skid on dry road, wet road, gravelly road, snowy road, or icy road?
On snowy surfaces I practice quickly winding/unwinding steering lock when the front wheels skid. Each time we pass from straight thru maximum traction to skidding it pulls the front end a little bit more. My daughters yell when I drive like that on snowy days, but then each of them have driven straight into poles and trees, and I have not. yet.

 

[LMF5000]

Tmoose: I was there under the car while the shop was doing the inspection. Going back on Monday for a more detailed look. He said I was lucky it was the wishbone that's bent. Didn't mention any other damage. I'm pretty sure everything was straight before as the car only has 7000 miles (bought it "used" - it was an ex showroom test drive vehicle).

Skid was on glassy tarmac on wet road (wet from dew, it was 1am). Don't think you have the stuff overseas, but here in Malta some roads are coated with this cheap shiny high-glass tarmac with very poor grip. To make matters worse this was a sharp hairpin bend up a 30-degree incline and the road was barely two lanes wide between kerbs. Course there were no road lights so in the darkness I didn't realise how far I was far from the apex, went to take the turn a little faster than I should have (well, at least I know that now...) and steering lost all self-aligning resistance - i.e. the front wheels lost all grip. No tyre skidding noise either. I instinctively applied full lock to the left and pressed the brake but there was not enough space to my right and the rim banged into the kerb. Then the car bounced left and the tyres regained traction, so now it was heading straight into the wall on the left, but I managed to correct that in time and get it straightened.

I've never driven in snow. I suppose the grip level would be similarly poor (considering I'm using summer tyres). Funny thing is I had taken this car to a small race track in Sicily and had no trouble going round every bend with tyres screeching (correcting the understeer with extra steering input). It's FWD so has a natural tendency to understeer when the grip limit is exceeded but it's very neutral and easy to control. It also has traction control, torque vectoring and the other modern driver aids, but since it was in "dynamic" mode (aka sport mode) at the time they were not operating as agressively as in "normal" or "all weather" mode.

 

[LMF5000]

Well, got the car back from the repair shop today. Replaced the lower wishbone (€68, shipped all the way from Latvia!) and had an alignment done (€70, including wishbone fitting). Car now drives straight with hands off and steering feels symmetrical. Only odd thing is that the steering seems to need more effort to turn than I remember. I have the alignment sheet and seems the shop used the databank values for the MiTo Sport (Quadrofolgio), whereas mine is a normal non-sport MiTo.

The left wheel was not impacted, so using this wheel's values before and after the alignment, I can see what they've changed:
Half-toe increased from -40' to -5'
Camber reduced from -17' to -34'
Caster reduced slightly from +2*56' to +2*40'

Would any of these account for the increased steering effort? Power steering motor is fine because steering is very light when parking, it's only with speed that it stiffens up.