Why Diesel???

[john_bud]

One question I have not heard about yet is how these gassers are performing so well, even though on paper, they should be getting blown away. I do not understand this fact.
JohnM

John,

It actually has been answered, several times and with technical back up.

The gas powered trucks use high RPM's and steep gear ratios to stay at the power peak while using stupendous amounts of fuel.

The diesel trucks use low rpm's and low gear ratios to stay at the Torque peak while using minuscule amounts of fuel.


Diesels are more fuel efficient due to higher energy content per unit volume and higher static compression ratio (18:1 vs 9.5:1).


Last point, why in the world do you think the gas engine trucks should be "blown away"? A 350 HP engine is a 350 HP engine. Run a few computer dragstrip simulators. It doesn't matter if it is a diesel or a gas engine, as long as you are able to shift at the correct point and have the recovery at the correct point. That is usually up shifting just after the HP peak and having the new gear ratio start pulling at the Torque peak. You do that, you will have the identical 1/4 mile times and trap speeds. Assuming traction is hooked up and the weights are the same and all that tiny font printing stuff. For trucks pulling a 10,000# trailer load up "9 mile hill" outside Albq (I used to ride my 15 speed bike up that hill for exercise!), that means the gas engined truck is upshifting at 5500 rpm and the next gear kicks in at about 4200 rpm. The Diesel is upshifting at 2600-2800 and the next gear kicks in at 1600-1700 rpm.

The diesels usually win those contests as they are able to provide the greatest average HP to the rear wheels. That's due to the lower HP drop between peak HP and peak Torque that a turbo diesel has. If you put a CVT behind 350 hp gas and diesel engines in trucks of identical weight, the pulls would be identical. The CVT's would have to be optimised for each, naturally.


jb

 

[unreconstructed]

Thanks for the encouraging words Rollingsfarms. Always wanted a diesel. I can afford one, to tell you the truth, but I just did not want to pay the price.
I have never even driven a V-10. Just going on what some folks I trust told me. Guess that's not too smart a way to buy, but what the heck.

 

[the_sandman_454]

I bought my diesel before the particulate filters, egr and other emissions crap was being applied. I bought it because I occasionally pull things around, I like having tons of extra power on tap, and because of mileage both loaded and unloaded.

I can pull my boat with my diesel Ram and get better mileage towing that than my 92 K2500 Chevy (5.7L and 4.10 gears) could get empty, and that's even accelerating faster with the diesel.

Diesels are fun with the power adders they have now too. Just tune your way to extra power with a few bolt on parts like a different turbocharger to help support the power. What other truck can you drive around that have the capability of putting down over 350HP and 750+ft/lbs of torque to the ground (although not quite stock) and still break into the 20's (a few trips where I've just set the cruise around 70 or so I've been knocking at the door of 21mpg). I don't think a tuned up gas engine in a truck would allow that sort of power figures with similar accompanying mileage.

And a tuned up diesel is handy to produce a black cloud of death out the exhaust to help deter tailgaiters and other idiots on the road. Usually a puff or two is all that's necessary to get them to reverse their rectal/cranial inversion and drive like reasonable individuals at a decent following distance.

 

[Z-Michigan]

Last point, why in the world do you think the gas engine trucks should be "blown away"? A 350 HP engine is a 350 HP engine. Run a few computer dragstrip simulators. It doesn't matter if it is a diesel or a gas engine, as long as you are able to shift at the correct point and have the recovery at the correct point. That is usually up shifting just after the HP peak and having the new gear ratio start pulling at the Torque peak. You do that, you will have the identical 1/4 mile times and trap speeds. Assuming traction is hooked up and the weights are the same and all that tiny font printing stuff.
***
If you put a CVT behind 350 hp gas and diesel engines in trucks of identical weight, the pulls would be identical. The CVT's would have to be optimised for each, naturally.
jb

Great idea on the CVT, and I agree 100%. I used to own a car with a CVT and it was really nice.

Just commenting since someone will ask about it - identical torque and hp won't automatically produce the same acceleration times with conventional gear transmissions. Engines use up some power in simply spinning up to higher speeds, which is affected by several factors but most importantly the total rotating weight (crankshaft + flywheel, mostly). Diesels often have very heavy cranks to stand up to the torque and very heavy flywheels to smooth things out at low rpm, which means a lot of diesels won't rev very fast even in neutral. This can make some difference in real-world acceleration. However, as practically every post has noted, the diesels will do better under heavy load because of their torque characteristics and the ability to use them reasonably at the RPM where they are making their peak torque and hp figures.

 

[Z-Michigan]

I bought my diesel before the particulate filters, egr and other emissions crap was being applied.

Amen to that. I think the 2006 Ram/Cummins and Chevy/Duramax engines are going to be highly sought after for a long time - massive hp but without the emissions burdens of the 2007+ models. The 2006 ISB doesn't have an EGR valve, which you definitely don't want if you can help it, and none of the 2006 and earlier models have the DPF filters prevent you from using higher sulfur fuel and that require complicated fuel programming (occasional rich burns to recharge the DPF) that hurts overall mileage.

 

[jcmseven]

JB,

I appreciate the clarification. I understand the logic but still have a question regarding its practice. Let's take the GM Duramax, for example, and the Toyota Tundra, both pulling a 10,000 pound load, say, for 1/4 mile. The Toyota enjoys a 15 horsepower advantage, an approximately 800 pound weight advantage and a 4.30 gear with a six speed tranny. Looking at the transmission ratios, they look fairly similar. Using this logic I would expect the Toyota to only enjoy its negligible horsepower advantage for a brief time. I would expect the Duramax to make MORE horsepower throughout the powerband, except at peak power. With the gas engine, I would expect peak power to come within a 500 or so RPM band. I would also expect the Duramax to make more torque than the 5.7L I force Toyota from idle to maximum. Although I have not seen dyno sheets I doubt the GM engine makes any less than 400 pound feet--ever. Assuming the torque curve for the Duramax is so much broader, I would expect the GM to get a heavy load moving quicker than the Toyota. I would also expect it to achieve maximal pull speed for this distance quicker and sustain speed better. For the brief moment the Toyota makes more horsepower it does not logically follow to me that it could make up more time and pass the GM Duramax. I realize that it is a higher revving engine but it seems the six speed tranny for the GM truck would negate any RPM advantage, i.e. in two trucks with fewer gears, it would seem RPM differences would have more impact on performance. Back in the days when I drag-raced cars, we would say 100 pounds of chassis weight equalled 0.1 seconds on the strip. Assuming a weight advantage of 800 pounds for the Toyota it would seem all things being equal, which they are not, that the Toyota would have a 0.7-0.8 advantage. It had the best time by 1.7 seconds, if I recall. Not that it matters that much when one is planning to tow a heavy 5th wheel or such for 200,000 miles, but to me it would be frustrating seeing a gas powered half ton leaving me in my $50,000 truck. Where have I missed the performance point?? I am assuming this applies to all the diesels. Also, there has been much made that the new post-particulate filter diesels get less mileage than their predecessors. I have personally not noted this to be true with my truck, but I have heard it the case with others. Why is it that a low restriction particulate filter would so drastically affect mileage??

John M

 

[radman1]

You guys are a sharp group. Most everything said is true. The newer diesels are getting hard to justify over the gassers. What % of people pull heavy loads daily, run very high miles and need the diesel as the ideal motor? Not too many people run in the 300,000+ mileage range. A cared for gasser should make 200,000 miles. Diesels are more expensive to repair and cost more to change oil, as well as oil and fuel filters. The decline of the diesel truck is probably going to happen in a few years with the increasing regulations such as dpf's and reduced nox emmissions and higher prices of diesel engines. Fuel mileage is dropping for the diesel. Diesel fuel prices are higher than gas in most areas. It will get worse in 2010 with the next round of regulations. Tier III diesel requirements will be hitting nearly all diesels in 2009 including CUTs, skidsteers, construction eqiupment etc. Manufactures are already struggling with trying to meet the new requirements. Next will be DPFs and so on.

Most people who own a diesel truck probably can't justify them on an economic basis. There is the prestige of owning a diesel truck.

Resale depends on the type of vehicle and its "perceived value". If a truck is desired by others, the resale is higher. Diesel have a perceived higher value and hopefully this will offset the initial higher cost. If the cost/performance of the gasser keeps improving, and the diesel desireability declines, the gasser will obtain higher perceived value and close the resale gap with the diesel.

Having said all of that, I have a 9 year old V10 but looking at a diesel for my next truck knowing that the gasser is probably the better choice for me. I have had 0 problems with the V10. I will probably decide in the next 6 months.

 

[radman1]

JB,

Why is it that a low restriction particulate filter would so drastically affect mileage??

John M

The DPF does increase back pressure which hurts performance. Pressure builds as more soot is deposited on the DPF. As sensors measure the pressure increase, a regeneration will occur to clean the DPF. During regeneration, extra fuel is required and dumped in the exhaust to reach the higher temperatures to burn the soot on the dpf to ash. The extra fuel required is hurting mileage. Many people report a regeneration about every 200-300 miles depending on driving conditions. DPFs are expected to require cleaning about 100,000 and replacement at 200,000 miles. DPFs currently cost about $1500-2500. Hopefully they will get cheaper as technology progresses. Now add that to the cost of your diesel.

 

[cp1969]

John,



The diesels usually win those contests as they are able to provide the greatest average HP to the rear wheels.


jb

There you have it, in twenty words.

It took me several hundred words to say the same thing only less clearly.

 

[john_bud]

John M.

It is confusing. There are more factors at play than I understand, so there are ones that I have overlooked, underappreciated or just plain don't understand. One that you may have overlooked is the function of the torque converter. At low engine speeds and high drive line loads, the converter slips and allows the engine to move up to a higher rpm. You may have heard about "stall speed" in torque converters? In drag racing, you get a converter with a high stall speed. At the line, you don't rev the engine against the converter (much), you let the engine "flash" to a high rpm just before the light turns green. The engine then is making snit loads of power and it is suddenly hooked to the transmission and WHAM! you go flying off the starting line and nearly snap your neck off. Down side is that your engine is always running much faster than the transmission as the converter is always slipping. Slipping makes heat, wastes power and results in a pure gas pig-hog-hippo. In my youth, getting a hot converter meant never losing at the stoplight, but only getting 6-7 mpg when cruising. YIKES!

With modern transmissions, you can have a fairly "loose" converter with a higher stall speed to help get the heavy vehicle moving from a stop. The trick is that a shaft slides in the converter and forces it to stop slipping for a 1:1 rotation (engine speed = trans input speed). That helps you get decent milage and have decent off the line accelleration. Gas powered trucks take advantage of that - a lot. They are tuned to run up to 6,000 rpm's for peak power and as a result they have poor off idle torque and really need the slip to raise the engine rpm's into a useful range. Diesel powered trucks don't want or need a loose converter, so they don't get one. They have the low speed power to move the load already and are fighting slip in the converter due to the massive torque. The gas engined truck's transmission is also going to unlock the converter for full powered runs. Another trick enabled by the computer age.

It's towing advantage to the gas engine in that regard, but at a cost. Much higher heat generation in the transmission and lower effiency (worse mpg).


The addition of the DPF will make the turbo less effective as it slows the exhaust flow and decreases the pressure delta across the turbo. That results in inlet lower pressure/ lower power/higher temps in the engine. To get the same amount of get up and go, the driver hits the loud peddle just a bit harder to get the accustomed acceleration. The greater the soot loading, the higher the back pressure and the worse things get. The DPF is cleaned by the injectors being told to squirt 22,000 psi fuel while the exhaust valve is open. It is then "burned" off in the DPF heating it up enough to also burn off the accumulated soot. Double whammy. You can't operate the engine as efficiently due to the extra back pressure and you uselessly squirt fuel out the exhaust so it can't be used to power the vehicle. Sure, it's only a qt or two of fuel every tank full - what's the big deal? I've gone thru 190 tank fulls in years and I've only gone 68k. (17.79 mpg to date). That extrapolates to 280 tanks in 100,000 miles. That's 70-140 gal of diesel squirted away, just for regens.


On edit - shucks, I forgot. If you really want the straight skinny - PM "kennedydiesel". He has a diesel performance shop and undoubtedly has forgotten more than I will ever know. He has his own in shop dyno too. Calls it a lie detector....