How come..

"nFamousCJ - Keeper of Stringbean, Gengars and a Deezul" (nfamouscj) 08/08/2016 at 18:27 • Filed to: None

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..diesels are more often turbocharged than super charged? Diesels make their power lower in the RPM range. Turbos make all their boost higher in the RPM range. Why not supercharge and avoid the turbo lag?

My citroen won't start > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:31

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I’m guessing fuel economy, turbos and diesels are there to save fuel, a supercharger would go against that.

Future next gen S2000 owner > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:35

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By altering the vanes and size of a turbo you can get them to spool much quicker. Negating some of the lag.

Cé hé sin > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:37

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My initial answer is that superchargers absorb more power but I’m sure there’s more to it than that.

The only supercharged diesels I can think of are two strokes and they’re not strictly speaking supercharged but rather use blown scavenging.

DynamicWeight > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:37

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You can tune turbo chargers to work at different RPMs by changing their size. I know I have seen a lot of dual turbo diesel engines which I assume have two different sized turbos to help out the engine at different RPMs.

Also, don’t superchargers rob the engine of a little power at the very lowest RPMs? Or am I tripping? This is a great question you’ve asked and I hope someone who knows weighs in.

Mattbob > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:38

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Diesels pump massive amounts of air. Think about it. diesels meter fuel, not air, so you are pumping tons of air, so it is really ideal for a turbo. Diesels also have a smaller rev range, so the turbos can be tuned to run well in that range. Overall, it is probably just more cost effective.

dogisbadob > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:39

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The turbo supplants the low diesel power at the high end. You have the low-end torque that the diesel can do by itself, then when the turbo spools up you have high-rpm power. It combines both.

And now you have twin turbos, where a smaller turbo spools up quick while waiting for the other, bigger, turbo to do so.

Jordan and the Slowrunner, Boomer Intensifies > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:43

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1. Turbo lag isn’t a huge deal in these.

2. NVH

3. The amount of boost the diesel takes.

Flavien Vidal > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:44

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Cost, emissions and low pressure turbos :)

If only EssExTee could be so grossly incandescent > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:44

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Efficiency. Superchargers always make boost, which is bad for economy when the engine is idling or cruising. A Turbo also ramps up the boost as revs increase, while a SC makes the same boost across the rev range so there’s a limit as to how much more work the SC will allow an engine to do.

Historically some large two-stroke diesels have been SCed but I think that had something to do with maintaining intake pressure rather than actual performance gains.

Alex from Toronto > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:44

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I think since diesels are lower rpm engines turbos are more effective then superchargers.

This reddit thread explains it better.

https://www.reddit.com/r/Trucks/comme…

RallyWrench > My citroen won't start 08/08/2016 at 18:45

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Haha there to save fuel

nafsucof > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:47

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Moar boost!!! Compound turbos make like 35-40 psi. Superchargerz would use too much energy to make that much boost!

Cé hé sin > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:49

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Despite what everybody’s been saying, supercharged diesels may become a thing.

My citroen won't start > RallyWrench 08/08/2016 at 18:54

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Fine, the land of string cheese uses diesel for something else.

Cé hé sin > If only EssExTee could be so grossly incandescent 08/08/2016 at 18:54

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So far as I know all two stroke diesels have had to be blown because they can’t use crankcase scavenging and can’t suck in air as a four stroke would. The blower’s main function is just to get enough air in rather than provide pressure (although they do that too).

TheHondaBro > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:56

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Exhaust drives turbochargers, not power. Turbos help supplement the top-end, while the engine itself produces adequate power in the low-end without the turbo.

HammerheadFistpunch > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:56

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its because its pretty easy to find a turbo that operates in the 90% efficacy map when you are only concerned about a tiny operating range of, say, 3000 rpm. plus, since the EGT’s of diesel are much lower compared to gas engines, you can design VG (variable geometry) into the design to reduce lag as they come onto boost. Lastly...and probably most importantly...they produce much MUCH more boost

RallyWrench > My citroen won't start 08/08/2016 at 18:57

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Hifalutin’ string cheese, my ass. This is Murica, sonny.

Die-Trying > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 18:58

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detroit diesel....... why settle for just one or the other?

bhtooefr > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 19:02

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Turbos don’t have to make all their boost at high RPM.

Smaller turbo, it makes boost at lower RPM. This is actually rather common on modern gas engines, too.

Diesels also have lower exhaust gas temperatures, and can therefore support things like variable nozzles to broaden the usable range of a turbocharger such that a large turbo works at lower RPM. (The 997.2 Turbo actually has this too, but it’s very uncommon on gas engines due to reliability concerns.)

Supercharging pulls power off the crankshaft, turbocharging pulls waste energy out of the exhaust, so it’s less efficient.

just-a-scratch > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 19:33

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An exhaust turbine braked by an intake air rotodynamic compressor (a.k.a. “turbo”) turns thermal and kinetic energy of the exhaust into potential pressure*volume and kinetic energy of the intake air. The turbo does work with the exhaust gas.

To compress the same amount of air to the same pressure and velocity via a shaft or belt driven compressor (i.e. supersharger), the engine has to output significant energy to the supercharger. The supercharged engine also puts out hot gas at some pressure, but all that hot gas is just vented to atmosphere without doing any work.

So, the arguments above are all about efficiency.

There are other benefits also. Diesel engines can be noisy, even small ones. The turbo has a tendency to smooth out pressure pulsations in the exhaust gas. As the gas moves over the turbine wheel, high and low pressure spikes tend to increase or decrease the wheel speed slightly. As this happens the sound pressure level decreases.

notsomethingstructural > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 19:42

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Enough with turbo lag, enough enoughenough. Unless you're running like a T04e on a 1.6L there is practically no lag because if there were no one would buy them.

jimz > nFamousCJ - Keeper of Stringbean, Gengars and a Deezul 08/08/2016 at 19:43

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Turbos make all their boost higher in the RPM range.

this is where your understanding is off. Turbochargers make boost based on engine load , not speed. if you’re lugging an engine down low in the RPM band, it’s still going to deliver enough exhaust gas energy to spin up the turbine and get on boost. Superchargers (except for centrifugals) are more or less linear devices, they move a certain amount of air per revolution and it goes steadily up with engine speed. a turbo’s ability to move air depends solely on the exhaust gas energy delivered to the turbine.

and while we’re on the topic, it’s turbocharging which gives an engine low-end torque (thanks to that characteristic of boost being tied to load.) DI turbo gas engines are matching or exceeding diesels in torque and hp for a given displacement. Diesel’s reputation for low-end torque is there because they’re immune to detonation and have been able to use high-pressure turbocharging for decades. it’s only in the last 10 years or so with the move to DI that gas engines have been able to crank up the boost.

jimz > Cé hé sin 08/08/2016 at 19:46

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you’re correct. A two stroke diesel (the most common of which in the US were the Detroit Diesel engines) cannot run without a blower. therefore, one with just the blower is considered normally aspirated. You can, however, add turbochargers too.

here’s a Detroit 4-53T, where you can clearly see the turbo connected to the Roots blower:

jimz > notsomethingstructural 08/08/2016 at 19:47

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people who talk about turbo lag (or turbos are only good at high rpm) need to stop watching YouTube videos of dyno queen Supras and Skylines.

notsomethingstructural > jimz 08/08/2016 at 20:10

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Agreed. Moratorium on talking about turbo cars unless you've owned one.

Steve is equipped with Electronic Fool Injection > notsomethingstructural 08/08/2016 at 21:13

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Meepster the Jeepster has turbo lag. But Fiat puts a pretty huge turbo on the 1.4 to make 180 lb ft of torque.

Seriously the jeep doesn't really pull until it comes into boost at around 2500 rpm

notsomethingstructural > Steve is equipped with Electronic Fool Injection 08/08/2016 at 22:04

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2200-2500 is close to normal but if you’re driving the car hard enough to complain about lag theoretically holding you back then you shouldn’t really be at 2500 rpm anyways. People make it sound like its VTEC and only works at like 5000 and above, which is much more of a legitimate gripe. The beef over turbo lag is ridiculous.

Steve is equipped with Electronic Fool Injection > notsomethingstructural 08/09/2016 at 08:00

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Nearly every weekday I have to merge into 55 mph traffic from a blind corner or a blind hill.