How reliable are turbo engines?

Do you enjoy spinning your tires on hard acceleration when climbing steep slopes in a FWD car? Or even when starting on flat surface?

In a ~250 HP FWD car torque steer is very obvious. Unfortunately, I know that from my own experience.

I think you must have missed the bit about throttle control. And I doubt that uphill makes any difference.

Besides, I've never experienced that problem.

Graham

Eeyore said in rec.autos.driving:

Of course it does. If you're heading uphill, the weight of the car will shift to the rear (i.e. AWAY from the drive wheels), resulting in less traction.

You're one of those "eggshell" drivers, aren't you? One of those guys who likes to pretend there is a raw egg between his foot and the gas pedal, and he presses the pedal gently so as to avoid breaking the shell of the imaginary egg?

Absolutely not actually. But I don't plant my foot right on the throttle when pulling away.

I did lose a bit of traction once accelerating heavily uphill in third rain in

3rd gear once at about 60 ish. Yes I know it was silly in the rain but I *was* in a heck of a hurry.

Made it to central London from my home town 20mi out in 20 mins !

Graham

Not on this subject. Mind you, I've over-simplified here and there, but still...

Huh? WTF are you prattling on about?

I'm saying that *my model* of car has been modded by folks to generate

500whp. The car already has a turbo. Folks start by putting on a larger-volume one. Clearer? 2.2L. This is not racing application either - street cars.

E.P.

Right. The mass is the important part.

In trying to give simple, short answers, I've got way off track. I don't have the time or desire to write a treatise on the thermodynamics of turbocharging, and in trying to be short and simple, I've really made a mess of it.

My apologies to anyone who cares.

If someone wants to know technical stuff about turbo cars, get the book "Maximum Boost" by Corky Bell.

I will stick by a couple of things - boost is related to rpms, when the turbo is doing work. (I'm not sure why we'd talk about it under other parameters.) While volume is not conserved, mass is. Including the mass of the fuel. And how fast that flows past the turbine determines how much air you're going to be able to cram in. (Yes, up to a point - the passenger car turbo loses efficiency at red-line rev ranges, in general)

Turbos are not put on cars to substitute for RPM, but for displacement. And they do recapture energy that would otherwise be lost.

Next time the subject comes up, I'll just post the wikipedia link, and let that suffice. Which is what I should have done in the first place. If folks want to pick on that write-up, no skin off my nose.

E.P.

And that's EXACTLY where you go off the rails. Boost is NOT related directly to RPM. Sure, there is a threshold RPM below which the engine cannot generate enough mass flow to fully spin up the turbocharger, but that RPM is *very* low compared to engine redline on anything except a heavily modified engine with a grossly oversized turbocharger for street use. Once the engine reaches that threshold RPM, the turbo is generating all the boost that it ever will, because at that point the wastegate will open (or the VNT vanes will move, or whatever mechanism is used) to dump further increases in exhaust flow past the turbine and let the reciprocating engine produce more power by holding the exhaust back pressure constant at the level that fully spins the turbine, rather than continuing to build. Yes, the engine will (up to a point) continue to increase its power output as you increase RPM, but its because of the exact same reason that a normally-aspirated engine does the same thing: power is porportional to force exerted*distance moved per unit time, and higher RPM is more distance moved per unit time. NOT because the turbo is continuing to work "better" at increasing crankshaft RPM. QED.

Turbos aren't a "substitute" for anything, really. They do allow smaller displacement engines to flow more mass and thus kinda-sorta look like a bigger displacement engine, but they still don't have the instant throttle response of a big-displacement engine. And they also allow an engine to have a torque curve that is less peaky and is filled in on the low side of the peak that it would have if normally aspirated, but the whole behavior of the engine really changes.

That's particularly true of the way diesels implement turbocharging. But most gasoline engines really don't gain a lot of efficiency from turbocharging because of the ol' detonation bugaboo. With a diesel, you can let the turbocharger do the lion's share of the work of compressing the incoming air charge, and there's really no limit (other than the strength of the connecting rods, block, gaskets, crank, etc.) on how much boost you can apply. Efficiency comes down to maximizing temperature and pressure ratios, and with a diesel you can do a LOT with a turbo. But with a gasoline engine, you're limited by detonation. Too much compression from the turbo, and you have to start compromising OTHER parameters (spark advance, fuel grade, add ADI, etc.) that lower efficiency themselves, drive up the cost of operating the engine, or both. Turbos on gasoline engines are for performance, but a Diesel without a turbo is more-or-less just a toy, excpet for very small applications like little generators, small tractors, lawn tractors, etc.

The reason I chose the word "related" instead of "proportional to" is very important. As I said, I'm *not* writing a usenet treatise on turbocharging. You spent a paragraph essentially agreeing with me and giving details.

Look at a compressor map for any turbo. The mass of exhaust flow is critical to getting the turbine spinning up to where the cold side is going to deliver max boost. Talking about the wastegate opening is sort of strange because that is not a function of mass flow, but a limitation set by the engine designers for several different reasons. The turbo that is on my car normally (from the factor) boosts to 2 bar (absolute). Changing the software to keep the wastegate closed can raise that up to 3.5 bar (absolute). None of the physical parameters changed.

Turbos do have a point at which they no longer can supply boost, that's true. But mass flow past the turbine is the starting point. And you can't get mass flow without air and fuel, and the faster you get those things burned, the faster you'll spin that turbine.

This is just being nitpicky.

[snip diesel]

Yes. One of the limiting factors for HP upgrades to my engine is the flow of the injectors, and the fuel pump. And most of the software for these cars does quite a bit of WOT fuel enrichment (11:1 A:F ratio at WOT, in some cases). In addition, the stock MAF will read out of range for flows that go 500whp, and the MAF needs to be upgraded. There are folks out there running nearly 4 bar (absolute) MAP on huge turbos that don't actually *start producing* noticable boost until after 4500 rpm, and make max boost at 7200 rpm (close to redline.) Now, on a 2.2L I5

20V engine, stroked and with upgraded internals, there's one guy out there pushing 1000whp. Not for the street, obviously. The choices are legion, and that's just for this particular motor type.

There are a wide range of choices to be made, which is why typing it all into usenet is a bit silly. AND, that's why I should NOT have started it in the first place. That and the fact that you'll get nitpickers at every stage.

E.P.

Your post reminded me of another turbo reliability issue (back to the main topic). A lot of lower end turbo vehicles do not have knock sensors. These motors can grenade pretty easily if fuel delivery is interrupted at high RPM, e.g. by clogged filter or bad injectors/pump.

Since when does any modern car not have a knock sensor ?

Graham

I'll go you one further- since when does ANY electronically-driven turbo car not have a knock sensor (and I'm going back to 80s Mopars and Buicks, here)?

I'm an engineer, I can't help being rankled by a choice of words that implied correlations that aren't correlations at all, just enabling conditions. The MOST you can say is that "a minimum RPM is required to deliver full boost." Beyond that, there is no correlation, proportionality, or really ANY other relation between turbo speed and crankshaft speed. There is, however, a strong correlation with turbo RPM and throttle position, and between turbo speed and EGT. Now that we've been around the tree 6 times, I guess I can say that "we agree." I think.

Hmmm. If there's more exhaust, the turbo is going to spool. And to keep it spooling, and delivering boost, you need exhaust gas flow. Other than turning higher RPMs (assuming some throttle position other than closed), I don't see how you keep the turbo delivering boost.

I see. That's why you get instant boost at 900RPM with full throttle. Ooops.

Throttle position controls RPM, in any given gear. But again, I'm not talking about intervals between shifts, of engine braking down an incline. During those times, the turbo is just along for the ride.

Well, we agree on all but the semantics of the discussion. I guess that's OK by me - I don't think we're going to completely agree on the chicken-egg portion of the discussion.

E.P.