Dave's point is more so an effect of having less backpressure, the motor wouldn't have to do as much work to pump the exhaust out. This would allow the motor to create the same amount of horsepower by not needing as much boost, therefore the amount of fuel used is decreased and the fuel economy increases.

I specifically remember in thermodyanmics class the professor stating that the higher the combustion temperature (after all this is what the compression ratio does) the more efficient the motor is. However our temperatures are limited due to the properties of the materials we use to build these engines. None the less, in our application by adding more boost you're also adding more fuel, which in turn makes more power than needed.

As the increased boost is raising the motor's efficiency it is also injecting more fuel. I can't see how it is getting any more efficient by creating this extra, uneeded power. You're also increasing the engine's backpressure in order to allow the compressor wheel to increase the boost since the EGTs are now lower (after all, turbos run off of heat and pressure differentials), this increase in backpressure puts more drag on the motor.

 

Hohn I followed you until the last statement where you got backwards. Yes, you got it right so far as the experts on big engines go. 16 to 20 pulling a hill. With the present 12 I could not achieve that. At half throttle I go to wastegate, which is wasted power through restriction. The slightest incline loaded part or half throttle goes to 30 plus and into the wastegate.

Going by what you said, the ideal would be the lack of restriction with pulling at the lower numbers. You with a 9 are cutting boost by restricting the exhaust. EGT's would go crazy and economy would be shambles.

You need to be looking at it from a different perspective. Running 68 to 70 and using the power necessary to pull the hill at these speeds, now configure the engine to get the best efficiency. Not slow to 30 and not use the power that the engine has.

So far as I can see from studying what those that know a lot more than I do, forget about lag, set the exhaust housing with no waste gate to the size that will produce the optimum boost and relax back pressure. That on my engine and loads would be a larger housing due to the fact that 30 pounds is not needed or for that matter wanted so long as the EGT's stay in line. I have a hard time getting over 1050 with the highest ever at 1100. The disadvantage to this is lag and the smoke that is produced before spoolup and the lazy starting power till it spools. Of course the lazy is not a factor with freight, only cruising the boulivard.

Something that you also skip is the fact that, rather lost on the diesel system, is the fuel to oxygen ratio that does count. Too much oxygen is not good just as too little is not good. On the old detroits we searched part numbers to find liners that had higher ports (German Export) so that the oxygen would be higher with larger injectors. Just the injectors lowered economy, with the liners and larger ports, it increased.

While the choice of exhaust housing size to "increase" to may be up for discussion, going to a 9 is rediculous for economy or power. What you are suggesting is just go to a smaller engine. My case is to get the 300 plus horsepower produced at a better effiency. Think about it, there is a big difference there.

 

HOHN, on the money, which by now I have come to expect from you.


BostdCTD said: "None the less, in our application by adding more boost you're also adding more fuel, which in turn makes more power than needed". and "As the increased boost is raising the motor's efficiency it is also injecting more fuel. I can't see how it is getting any more efficient by creating this extra, uneeded power. "

This is not the case. There is no un-needed power. IF you are making more power you are going faster. Just because the boost is increased does not mean more fuel is being injected. Fuel rate determines the power output, and exhaust housing then determines what boost is made at that power output. Increase the fueling rate with a given exhaust housing and you increase the boost, not the other way around (except when you look at fueling boxes which add additional fuel based on boost pressure, but this then makes more power so you just go faster)

Increased power output goes into one thing, increased speed the vehicle. Increase the power and you increase the speed. There is no such thing as driving at 60 MPH using 100 HP vs. driving at 60 MPH using 150 HP because of injector changes or turbo changes or any mechanical change for that matter. Power required at a given speed is only a function of wind, rolling resistance, driveline friction, mass and grade, that’s it. Lets say a truck pulling a load up a grade at 60 MPH uses 100 HP. Now you change the turbo, injectors, timing, exhaust system... Guess what, go pull the same load up the same grade and you are still using 100 HP. Just now you have more power left under the pedal if you desire to used it and you probably have altered your efficiency while pulling that grade.

'Driving by the boost gauge' just means you are keeping an eye on your power output and controlling it.... This is why you are seeing better mileage, not because of the 'boost control' of your foot, but the power control of your foot. Measured boost just happens to be an indication of power delivered, all other things being equal.


HID: Yes, if you are regularly seeing 30 PSI and opening the waste gate you could benefit from a larger housing indeed. Once the wastgate opens, things get inefficient in a hurry. You are correct, for towing efficiency get a non-wastegated housing and set the housing size that gives you about 35 PSI when full tilt to the floor and you will be doing about the best you can.....

This discussion once again highlights the need for VGT (variable geometry turbine) for those who are serious about their performance and efficiency. A non-wastegated VGT is as good as it gets for all around use.


Kevin

 

Kev beat me to it.

HID-- your power requirements are beyond what an Hy-9 can deliver. What I was implying that swapping an HY for an HX35 on an otherwise stock truck SHOULD net a small efficiency gain.

I'd go so far as to say you only want about 28 psi at your heaviest pull. If you run over 30, then the housing is too small.

For guys who have increased fueling to where the stock charger is no longer efficient (i.e. running the stock charger at 40+), a larger housing will almost ALWAYS mean an increase in fuel economy.

The problem is that the lag associated with 16 or 18 housings often leads to smoke, which hurts MPG.

With your usage profile (and the fact that you are ALWAYS towing, and don't drive like an Indycar driver), I'd say a larger housing is at least beneficial, and maybe even a slighly larger charger.

Holset says that the HX35 can only support 280hp.http://www.holset.co.uk/files/2_1_1_2-HX35.php

From the sound of things, HID-- I think you'd be a candidate for Holset's new Super 40. Your airflow needs are right in the HX40 range, and the Super 40 has a higher Pressure Ratio compressor wheel to account for the fact that we are using it on a smaller engine (5.9 instead of C8.3)

JMHO

 

<edit> Nevermind, the system works different than what I was thinking and I'm used to. My M/B has a pressure switch (ALDA) which is adjusts the fuel by boost, but no fuel plate on my Bosch. The Cummins on the other hand just rides the fuel plate once 6psi of boost is hit. I had the theory wrong with the variable valet switches, but it is just stopping the fuel plate coming into play or in case of a variable one it is just delaying the response.

 

HOLY COW, I think I'm feelin a little smarter just bein in the pressence of a conversation like this. Guess I shoulda spent a little more time in the school, maybe a little less chasing the ladies.

 

Three questions for the combustion experts.

'89-'93 CTD's had 18.5 and even 21 cm turbine housings. In 1994 they switched to the 12 cm wastegated housing. The power rating stayed the same. Was this done for fuel economy or emissions?

Does maximum combustion efficiency=maximum fuel economy?

Off topic but related: Does maximum combustion efficiency=maximum power at the flywheel?

 

Is that the same question as:
Is the point at which your wastegate is tuned to give minimum EGT (same turbo, same injectors, etc) the same point as maximum fuel efficiency or maximum power?

When do you tell me how to get 600hp with my stock turbo, stock injectors and stock air filter? OK, maybe a 4" exhaust, BUT THAT'S IT!

 

1)neither. It was done because of driveability complaints. The fact that it helped emissions was bonus. There were no new emissions standards to be met in 94, to my knowledge. The quicker spoolup would probably reduce smoke and increase efficiency by a small, perhaps imperceptible, amount.

2)Not necessarily. As aluded to before, you can have an efficient engine still have poor MPG if it's being asked to make a lot of HP. Given constant HP demands, the increased efficiency will help MPG, but the MPG issue is MUCH bigger than just marginal efficiency improvements.

3) Almost never. The point of maximum "efficiency" is usually peak torque. This is the resonant point of the engine, where volumetric efficiency is highest and BSFC is lowest.

In a CTD, maximum efficiency will come when both the engine and it's turbocharger are in their respective sweet spots. For a 24V, the sweet spot is 2000 rpm. The engine has its lowest BSFC at this point, so it makes the most HP for a given rate of fuel consumption. Now we add to this scenario the efficiency of the turbocharger, so let's say it's making 20psi of boost.

In the above scenario, the engine is spinning 2K rpm with 20psi of boost. It is most efficient in terms of HP produced vs Fuel consumed.

Now, is this going to give you good MPG? NOPE. Clearly, the engine is having to make more HP and burn more fuel to sustain 20psi than it would with 2K rpm but ZERO BOOST.

So the point of max MPG will come at the point where the trans gearing is keeping the engine within it's BSFC range while allowing a speed that minimizes the drag effects of tire resistance, wind drag, and driveline losses.

Thus, best mpg is probably in 5th gear, between 45-50mph (for an NV5600, 4th for an NV4500)

You could plot the BSFC curve of the engine, then plot the RPM for each gear and the "road load" factors (i.e. rolling resistance, wind drag, etc) and find the maximum MPG speed for any combo of tire size and trans ratio.

For most cars, 50-55 is max mpg, but since our trucks have the aerodynamics of a brick wall, the optimum speed is slower (shifts road load curve to the left).

I'd be willing to bet that I could get a 2wd ETH, DEE reg cab truck over 30mpg with a camper shell of my own design, some Airtabs, and a constant 45mph in 5th gear.

Justin

 

Now something finally makes sense to me. I put on probably 400 miles while my 5th gear was out of my NV4500 and I was amazed at the differance in fuel mileage, on both highway and around town. I never went over 55mph in 4th, so it wasn't fast, but I bet I gained at least 3mpg over that time

 

Ok so with the VGT turbos, what range do they have for airflow. How do twins come into effieceincy? What about twins with dual VGT's??

Hohn, what would be ur design of a camper? Need a test truck??

I think I should be getting mpg that what I get now. 14 in city. however when towing 23K gross, it never dipped below mid 11's even on the Grapevine, but I drove with a light foot. Didn't have gauges then and the truck was completely stock.

 

VGTs with twins can only help efficiency. I don't know much about them other than what I have read. I've been told that a VGT for our trucks could get as small as an Hy-9 (and spool in a blink), while still flowing enough air on the top end to support 450-500 hp.

Twins help efficiency by dividing the workload between two turbos. Each is more efficiet at a reduced workload. Say you need a total pressure ratio of 5:1 (or 58.8 # of boost, given atm pressure of 14.7 psi). With twins, you only need each turbo to have a 2.23 pressure ratio (which is the square root of 5). It is generally MUCH more efficient to operate two turbos at 2.23 PR than one turbo at 5:1 PR. This is because it's just really hard to design an efficient compressor at 5:1 ratios!

Ever notice that most industrial air compressors are two stage? This is for the same reason. If you want to fill a compressor tank to 175psi, it's much more efficient to divide up the workload between stages. It uses less electricity than it would to have a single stage trying to push 175 psi.

Twins with dual VGTs would be SICK. It might also be a waste, because the whole point of VGT is to give you most of the benefits of twins with more simplicity and reliability. It also takes up less space.

That said, twins with VGTs would allow you to have a much greater size differential. This allows some interesting possibilities. Now, your small charger could be even smaller than an Hy-9-- giving instantaneous boost. Your large charger could be HUGE and still spool well. Thus, you could expand both the bottom end (faster spoolup) AND the top end (more airflow).

A twins setup that spool instantly and will support triple digit boost levels? Sounds good to me!


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Aero aids could greatly help our trucks. The camper shell of my own design would focus primarily on aerodynamics. The idea here is to transition from the flat trailing edge of the cab roof to the top of the tailgate while ALSO transitioning in the SIDES of the cab as well. It would end up looking like a ramp going up from the tailgate to the cab, but more of a bubble shape.

This still leaves you with the flat rear of the truck to deal with. A bubble shape here, somewhat resembling a cone, would greatly reduce pressure drag. Think of the cone-like aero aid used on the Space Shuttle when it rides piggyback on the 747.

In aerodynamics, the trailing edge is MUCH more important than the leading edge. It's a little counterintuitive, but it's true. if you have a strong wind blowing over a triangle shape, the MOST aerodynamic way is to have a blunt side of the triangle facing the wind, with the sharp point facing away from the wind.

Adding Airtabs would help, because they trick the airflow into thinking that there is a bubble-like shape at the trailing edge of the vehicle.

Sorry to ramble, and I know it's way off topic, but the question was asked.

jlh

 

I've always been interested in aerodynamics, here is where NASA took a van and bastardized it to reduce the aerodyanmic drag. Some U of M students did a rough approximation of different setups with this clay model in a wind tunnel. This page describes the effect of the coefficient of drag (Cd) on vehicles. Here is a list of popular vehicle Cds. I thought it was quite odd that a Dodge Viper has more aerodynamic drag than most of the other Chrsyler line up, it has a worse Cd (according to that page) than a Dodge Ram SLT.

Unfortunately though, I don't think HiD can really lower his aerodynamic drag on his towing rig. Have you ever tried the TAG, HiD? I heard it mainly helps people who tow a lot, makes the flow go into the turbo more laminar so that it makes the compressor wheel more efficient.

 

No drag is the worst on a car hauler. Sometimes I have a full sized van backwards on the front of the trailer, which puts the van completely over the roof line of the CTD. Flatbeds are bad but the wedge car hauler is the worst. All the structure of the trailer in a wedge design, plus the load over the top in front. Also is the 4wd bumper line which is bad, in the future I plan to get air dam lower to help that. I have been thinkin of the TAG and it is on the list of priorities. I gues in a perfect world, you could plate the frame of the trailer and help some, especially the sides and bottom. That is why I call it a parachute, it acts like one. On mine th eslightest amount of wind, from any direction kills the power and economy.

It is also interesting that the original engine would bog and I could not maintain freeway speed with a sizable headwind, but now that is not a problem. Believe it or not, with the original engine, bogging into the wind, the economy was much worse than the present engine running 70 in the same conditions. Foot flat on the floor for extended periods distroys any economy.

I have watched the threads, especially Scotty, on the TAG, and I plan on trying it. Need to do some exhaust work first, at least one stack is necessary, and it has to run beside the cab at the rear, so I guess will be called a "red neck" stack, but mine is not for looks or style but for practical purpose. There is not room for the stack behind the cab in a 90 degree jack with the trailer.

 

mabe it was said here allready-missed it, there is a new generation of turbos on the way and some being produced now. They have vanes that adj. as required-lot less room for error as it covers low, high and midle grd. To top it off they are self contained (no oil problem no wast gate)
Preettty $$ at the moment