Fuel Economy Modifications

[nick]

What kind of fuel economy is everyone getting?

Have any of you tried improving it outside of changing your driving style?

I haven't had my car for more than a couple months. It has had a blown head gasket since before I got, but I have been getting 38.5 MPG. I just replaced the head gasket and haven't had the opportunity the check my fuel economy with solid combustion chamber seals.

I want to increase the thermal efficiency of my motor. My EGT cruising down the interstate is at least 600 degrees F. I'm thinking about using a full sized muffler off of a Powerstroke or Duramax, gutting it and building an air to liquid heat exchanger inside it to preheat the fuel going to the injector pump using exhaust gases. I'll be installing an electric lift pump near the tank to overcome the head loss incurred by the extra long path the fuel will have to follow.

My purpose is in attempt to recover some of the waste heat of the combustion process. I figure if I can raise the temperature of the diesel fuel 200-300 degrees before it enters the combustion chamber, then I'll need less mass of fuel to produce the same about of heat in the combustion chamber. In gas turbine applications this is called a regenerator. The reason I want to use a giant muffler off of a much larger vehicle is because I want to minimize the amount of back pressure the heat exchanger causes, otherwise the device would be pointless.

An alternative, much easier design would be to use the coolant running through the block to preheat the fuel with a liquid to liquid heat exchanger. The only problem with this is that I would have a peak fuel temp of just below the coolant temp, ~ 170 degrees.

Hopefully I'll have a chance this week to run the numbers through the calculator in order to figure out the length of 3/8" ID stainless steel tubing I'll need to bring the diesel from 70 degrees to 300 degrees. Just to clarify: I'll be transferring heat from the 600 degree exhaust air to the 70 degree diesel fuel using a large diameter muffler that has multiple feet of stainless steel tubing inside it.

Does anyone have suggestions or questions?

[v8volvo]

You must be an engineer, or at least someone with a mind for energy transfer and thermodynamics. :cool:

My D24Ts usually hover around the high 20s in town and low-to-mid-30s on the road. My daily driver, an '86 745TD with original 280k on engine and automatic trans and getting a little tired in both places, still gets about 26-27 in mixed commuting. My manual 745TD got 32-34 on a trip cross country fully loaded. My manual 764TD (an '83 like yours) got ~33 on a heavily loaded trip from Seattle to San Francisco and back over the summer.

I track all my vehicles' fuel use at Fuelly.com, which is a great site for recording, synthesizing and sharing MPG data. You can see my fleet's page here:
http://www.fuelly.com/driver/v8volvo

I recommend it as an easy and effective way of keeping track that others can see too.

I think running the least restrictive exhaust possible is always a good idea, especially on a turbodiesel engine. My eventual plan is to fit a VNT turbo (I have a Garrett GT2056V off a Liberty CRD sitting in my living room), which I think will harness a lot more exhaust energy and boost overall MPG significantly. Heating the fuel is always tricky, since some heat is good but you definitely don't want too much for the sake of the IP. I think I would work with the exhaust and turbo and do some intercooling before I tried heating the fuel.

[nick]

Thanks for the resource!

I used to have a 12 valve intercooled Cummins with a P7100 and 3.5 inch exhaust with a Banks Engineering muffler. I put 40k miles on the truck in a year. The only way I could get better fuel economy is by driving slower. The 4wd truck with 35 in tires, 3.55 ratios, and a nv4500 5spd always got 20 mpg running 75 mph. I even put 4" straight pipe exhaust from the turbo back (hoping for improved fuel economy) and gained no change in fuel economy. With this foundation, I see little to no value in freeing up the exhaust in my Volvo.

You definitely make a solid point on the wear I could put on the IP by running hot diesel through it. I'll have to consider the effect of that even though the fuel will gradually reach that high temperature. So, one good thing is that the IP wouldn't be in shock, but it would be gradually introduced to the hot fuel.

[Jason]

I'm with George about the hot fuel in the IP, probably not a good idea, the ip is cooled by fuel after all. I don't see a huge difference in economy comming from raising the fuel temp from what it is by the time it gets to the injector when the engine is hot under normal conditions, up a couple hundred more degrees, it still has to raise in temp by many hundreds more in the prechamber. Also, having had a few diesel trucks (dodges, fords, etc...) there is NO WAY your gonna fit one of those mufflers under the car! There just isn't any space. Maybe out back behind the axle, but it would look rediculus. As for free flowing exhaust, generally a better exhaust will net better economy, but it depends on the engine. Your 12 valve cummins with a 3.5 inch exhaust wasn't all that restricted if you were talking about straight highway and low fueling. I would have expected a combined city/highway increase though, when the engine is working harder on acceleration. Take that same exhaust and put it on a larger engine like a 7.3 powerstroke, or a later 24 valve cummins, and it becomes more of a restriction. On one of my previous trucks, a 99 F350 powerstroke, I gained almost 2mpg average just by straight piping it (getting rid of the muffler and cat).

My recomendation would be this: advance the ignition timing to right at 1.0mm, remove one of the shims under the cover held on with two screws on the engine block side of the pump (this is the pump timing advance spring, there are three shims, two at one end of the spring and one on the other, remove the thickest one leaving one shim at each end of the spring. I wouldn't do this unless you have the pump off already. Its not going to be a huge difference but every little bit helps). Rebuild your injectors, and set the pop pressure to the high side of the spectrum, around 2200psi if my memory is correct. I would recomend the later stock turbo diesel nozzles, DNOSD 293s. Our engines stock had the 193s, which were also the stock nozzles on non turbo D24s and 1.6 engines. The 293 is *supposed* to have a better/updated spray pattern, since the nozzles were refined and improved over the years. I would also at least remove the two stock junk mufflers and install a straight through muffler if your not a fan of the louder exhaust. Even without a intercooler, the NA manifold would also be a nice addition, the longer intake runners help low end torque and with conservative driving I think that would benefit mileage as well. Also the smoother intake path would be helpfull. 38mpg is already better than most are getting, and a 7 series is quite heavy. I only averaged 42 mpg with my jetta TDI, if you can get 40+ out of a heavy-ass IDI Volvo, thats impressive!

Jason

[casioqv]

I wonder if a hotter thermostat would improve thermodynamic efficiency? On Mercedes diesels some people report a significant improvement in economy going from an 85C to 95C thermostat. As long as your cooling system is up to snuff and keeps temps below 100C, I think this would be safe. A full length skid pan under the car, and leaning out the engine via the "smoke screw" probably would help also.

I don't think I would want to do such modifications on a D24T (making it run hotter and slower) but I'm just brain-storming of easy ways to improve economy.

[nick]

I think you are correct about using a hotter thermostat. I know guys do that with the 5.9 cummins trucks.

I'm not sure you'd be able to make the motor run any more lean on fuel since it is fuel regulated as opposed to "throttle" or air regulated on a gasoline engine.

I think that I'm going to build a coolant heat exchanger before I go all out and build the exhaust air heat exchanger. My two reasons for this are: simplicity & overheating the injection pump.

[Jason]

If your really hell bent on heating up fuel, have you thought about the electric heater wrap thats made for injector lines? They make those kits for running veggie oil to help heat the fuel, you could intall something like that, its on the injector lines so it won't be heating fuel into the pump. I don't know how hot they get but its enought to heat up the veggie oil to help get it to flow in cold weather. I have seen them on a few websites, it might be worth looking in to.

Jason

[casioqv]

Have you done any thermodynamic calculations on how much efficiency will increase? How hot do you plan to heat the fuel?

As others have mentioned, the fuel system isn't designed to operate with superheated fuel.

#2 diesel fuel has a flash point of 143F (62C) and an autoignition temperature of 410F (210C). Possible risks to the injection pump aside, if your diesel fuel is very hot it suddenly becomes an extremely flammable fuel whereas the injection system is designed to operate with a relatively non-flammable fuel and sometimes experiences small leaks. I wouldn't personally recommend heating the fuel above it's flash point, but perhaps you could use biodiesel which has a significantly higher flash point.

Many diesel engines (like the OM602 Turbo in my 190D) have thermostatically controlled fuel heating systems to prevent gelling, but I've never heard of heating fuel for reasons other than gel prevention.

I'm skeptical that any safe and practical fuel temperature increase will yield any significant increase in efficiency.

[Jason]

I agree, I think there are plenty of other areas of improvement that should be looked into first, such as the things I listed... Besides that, most of the other things are much less labor intensive than building a fuel heat exchanger setup.

Jason

[casioqv]

I'd really like to make a speed vs efficiency plot for the D24T like they've done for TDIs over at TDIclub:

http://www.drive55.org/downloads/tdiclubchart.gif

I don't know how accurate that one is, but I think one could make an extremely accurate one using a flowmeter in the fuel line, looping the return line back into the supply line after the meter, and using a GPS for precise speed measurements. I *suspect* that D24Ts easily get 40mpg if you drive slow enough at a steady speed on level ground, but the temptation to drive faster is too great!

NMEA2000 output fuel flowmeter:
http://www.jandhproducts.com/zoom_img/_1268745712.gif
http://www.jandhproducts.com/_e/Fuel_Monitoring/product/Lowrance_EP60R/Lowrance_EP_60R_Fuel_Flow_Sensor_120_39.htm

//edit: Dangit, apparently that meter is gasoline (not diesel) only. Diesel ones seem far more expensive :(

[Jason]

Getting 38mpg as the OP already said is pretty impressive as it is. These cars are much heavier than a golf or jetta TDI. I'm sure straight highway cruising could net 40mpg, depending on speed. There are many variables though. The TDIs seem to be pretty consistant in mileage from what I got with the 99 I had, and what I have read and talked to other people about. The volvos are much older, some much more worn out than others, some manual and some auto which seams to change the mpg quite a bit, etc... I got 34mpg out of mine cruising an average of 70-75mph on a trip up to chicago. That was with the old pretty much worn out engine. I have yet to do a mileage test with my new engine, but I suspect it will be up quite a bit.

Jason

[casioqv]

Our cars weigh roughly the same as a golf or Jetta TDI. The factory curb weight for a 1984 760GLE is 3190-3217 lbs. A 2009 Jetta TDI is 3364lbs (http://blogs.insideline.com/roadtests/2009/06/2009-volkswagen-jetta-tdi-track-tested.html), and a 2004 Jetta TDI is 3148lbs (http://65.110.12.166/showthread.php?t=206984).

The 740s are considerably lighter than the 760s. I've weighed a gasoline 1987 744 Turbo at 2850lbs.

The D24T just isn't as efficient as the 1.9 TDI.

//edit: I could never get more than ~30mpg highway from the 4 D24s I've owned but I usually cruise 75-85mph.

[corey_rhymes_with_orange]

I get around 40 mpg in my 82' 245 NA diesel, when I run straight diesel. It has a manual trans, 350,000 miles but the engine was rebuilt around 300,000. Oh and my IP leaks a fairly steady drip so my calculations are a little skewed. :p

[bryancald]

I'm averaging 25-27MPG, mostly mixed driving with a slow to engage overdrive (about 30 freeway miles before engagement).
I'm a little disappointed, I thought I'd get more like 35MPG from this engine (N/A D24). It has ~158K on the clock.