I'd like to throw in my 2c..

Turbo ratings from the gasser world will not work for the diesel because we have a completely different A/F ratio than them.
On the turbine side in a gasser you have a mass flow that's almost proportional to the hp produced. In a diesel you have a mass flow that's close to proportional to boost x rpm, the amount of diesel we inject is comparatively small and while going from 20% throttle to 100% throttle the mass of the exhaust gas doesn't change significantly. The mass flow only starts to change as boost and rpm rise. In a gasser with it's throttle we have an instantaneous reaction in the mass flow as soon as we change the throttle position, and for low throttle situations even a spooled turbo will produce boost against an obstacle so the tuning of a turbine and compressor is quite tricky. In a gasser you will normally not have high pressure ratios and will be interested in a compressor that has a rising efficiency and quick rise of mass flow as the boost is falling due to opening the throttle. In a diesel you want it quite the other way round. You need high pressure ratios, you will want a quick rise of mass flow over a rising pressure ratio when the enthalpy in the exhaust gas rises. So the turbine design for a daily driver will be quite different. For a dyno truck or a puller etc you will be accepting tradeoffs that you couldn't accept in your daily driver. To me tuning a turbo on a diesel is a quest to get the widest powerband possible, with the least amount of lag and the quickest possible transition (to reduce smoke and lag) with as little backpressure as possible and while staying within the EGT limits. Then I still want to optimize for fuel consumption.
These factors will dictate the selection of a turbine and compressor trim, and they will change masively depending on the use the engine will see. For me I usually cruise around almost empty, no trailer, at speeds of 60-85 mph,the only load being drag. So my compromise to get good driveability is a very small exhaust housing (with a very big wastegate) and a medium sized compressor that's just above the surge limit in these situations. As soon as I step on the go pedal the rising enthalpy spools the turbine and the compressor raises pressure (rpm don't change that fast, there's a truck in OD, converter locked that has some inertia inhibiting that), in turn I get an increased mass flow at the almost same rpm and get a: more mass flow on the turbine and b: enough air to burn more fuel which then increases enthalpy even more etc. I think it's crucial (for driveability) to always have enough "reserve air" to be able to ramp up the fuel quickly to avoid lag.

I would like to get into detail further but I should do some work too :-)

 

Man, this is a great thread.

How much does the injection timing affect enthalpy?

 

Well, at first blush, more fuel timing should increase the enthalpic rate of ascent.

 

Alpine-- you are correct in the mass flow variance between gassers and diesels, as far as I can see.

But we have to remember that it's not JUST the mass flow to the turbine-- it's the ENERGY. So a smaller mass at higher temp will drive the turbine as effectively as a larger mass at colder temp. This is why the turbine maps show "corrected" flow. It would be more accurage to go by drive energy (enthalpy), but corrected gas flow seems to approximate things closely enough.

Gas engines have relatively constant EGTs, but wildly variable amounts of heat (thermal energy) because of the variable mass flow resulting from air throttling.

Diesels have variable EGT and and wildy variable amounts of heat (thermal energy) at a relatively fixed mass flow (assuming constant RPM and such).

The end result in the eyes of the turbine is about the same-- though they take two different paths, the gas and diesel meet at the same destination (more turbine power and hence, boost).

I think Alpine said as much, but I'm trying to clarify my understanding of it.

The idea of "reserve air" is a VERY important thing, imo. A gas engine has no problems with reserve because opening the throttle farther gives the engine instant access to more fuel AND more air (unless you are already WOT).

But a diesel has only a certain reserve amount of air that is partially determined by turbo size and A/R. When you want more, you can only use up the air you already have, and past that your fuel becomes smoke. So the amount of "instant" response you have is limited by the surplus air-- after that you have to wait for the turbo to spool.

Unfortunately, the more "surplus air" you have, the more inefficient your engine becomes-- higher pumping losses, etc-- and the drive pressure will get very high quite early.

The "ideal" turbo setup for efficiency would hold EGT at a constant 1100-1200, providing just enough air to prevent meltdown. That would be very laggy too.

It's all about which sets of compromises a person can live with. This will vary a lot from person to person and truck to truck. The same guy who can tow at 450hp with a Silver Bullet on a 3rd gen HPCR manual trans would probably not want to drive that same turbo on a 400hp 12V automatic!

Justin

 

You can't quantify this very easily because it varies so much. It's COMPLETELY non-linear. The first degree of advance or retard might not have much effect. Two degrees will have MORE than twice the effect of 1 degree. Six or seven degrees might have 100 times or more the effect of that first degree change.

There's more to say on this, so I'll try to post more later...

jh

 

The enthalpy of exhaust is a strong, nearly linear function of temperature. Any timing move which would increase the EGTs will increase enthalpy. So in this regard, retarding the timing would increase the enthalpy, since the fuel burn is later in the cycle and under certain circumstances may still be burning in the exhaust. There might be something said about initially retarded timing until the turbo is lit and then crank in the advance later. This of course is only easily practical for us 24V electronic injected guys !!!. I actually think this might be precisely what Marco is doing to get the turbos to light so quick with the MAD and Smarty.....

So glad to hear us talking about enthalpy. No turbine conversation is intelluctually honest without understanding enthalpy!!!!!

PS: Check out the photos of a 1,000,000 HP/ 1,400,000 Ft*Lbs (that is not a a typo) turbine in my photo gallery, for an extreme example of extracting horsepower from enthalpy!!!! lol


Kp

 

I've heard the same thing with regards to Smarty. Now we just need him too write a different program for each a/r...

 

I know the turbo Buick guys have been doing that timing trick for awhile-- retard to light the charger, then bring in the advance. On a diesel, retarded timing with a healthy shot of fuel will light a turbo pretty darn quick!

Even though the relationship of EGT to enthalpy is linear, the relation of timing to EGT isn't, so I can't figure out a way to quanitify the effect of timing on enthalpy other than reiterative Trial and Error..

 

Couldn't we use a data logger of some sort, thinking about the gauges you posted about before. We should be able to plot the rise of EGT with in a certain rpm range.

 

Here are the maps for our single turbos. It compares them to a stock hx35

 

The graph says HY35, smaller turbine housing.

 

If people here need the moderators to protect them from the real world, I think turbos are the least of your problems.

 

Iron Horse,

If it's not too much to ask, please keep this thread about turbos. We welcome your input on the subject.

thanks,

 

I agree,,,,,, this thread was started on the topic of turbos and I think some folks need to back off on the sarcasm just a tad.

We are all here to learn,,,,,


Thanks

 

On my truck, advancing the timing with a healthy shot of fuel will light the turbo faster then using the stock timing. Remember until you are making boost, it a normally aspirated engine with an exhaust restriction. For max boost however retarding the timing on my truck will make more boost, but hp drops as drive pressure goes up.

Paul