Brad4321
Jr. Member
Posts: 75
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« Reply #28 on: November 30, 2009, 08:00:12 am » |
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I am a diesel mechanic by trade (and engineer on the side), specifically work for cummins currently, but I only own international. I am not going to claim to know everything, but I would like to comment on a few things mentioned in this thread. I have not ran hydroxy through any of my diesels (testing on my small ranger), so I can't comment directly on that yet. Nearly all of what I am mentioning below is engine THEORY. As we all know, theory is not always actuality, so keep that in mind.
Air/fuel mixtures are completely different in a standard diesel (there are exceptions, especially newer and foreign models) than gas. Diesels do not have a throttle valve, so they do not meter air in any way*. The engine throttles itself (controls rpm and torque) by fueling. Need more power? Dump in more fuel. More fuel creates more heat which spools up the turbo dumping in more air. Here is the key difference between diesels and gas...gas engines run hotter the leaner they are, diesels run hotter the richer they are.
Side note: I have been reading this forum for about a month now getting ideas for my generator...and I have read that a few people have been leaning out their engines in an attempt for better MPG. This is a wonderful idea except without an EGT pyrometer, you have no idea what your exhaust temperatures are. Leaning out a gas engine increases combustion/exhaust gas temperature to a point where you could melt a piston/valve. EGT gauges/probes are not cheap (anywhere from $100-$300 depending on options), they are a near requirement if you are wanting to mess with your fuel mixture...and they have many other uses that you can read about online.
It is really hard to "lean" (injecting less fuel since air is unmetered) out a diesel. It takes a certain amount of power to accelerate, climb a hill, or whatever. A diesel makes that power by injecting the perfect amount of fuel for the given situation. If you lean out a diesel, you are simply making less power. Diesels do NOT run at stoichiometry or any of the nonsense that gas engines do. Under cruise, it is common to find a diesel running about 100:1. By the time you get a gas engine near 20:1 you are really close to burning a piston or valve.
Simply put, diesels already run lean. Black smoke is running rich. A slight gray haze out of the pipe is the perfect fuel combination for maximum power. A thick black smoke is just wasted fuel. If your diesel never hazes out smoke, you are not making anywhere near maximum power. You get better efficiency with a smaller engine at full load than with a larger engine at partial load, keep in mind.
Diesel fuel burns slower than gas. When atomized, we are talking millionths of a second, but it does burn slower. This is one of the reasons diesels tend to turn less RPM than gas engines on average; you have to allow for more time for combusion. Hydroxy would tend to speed that up...to a very limited extent. All modern diesels have an injector directly into the cylinder. The injector sprays (upwards of 20,000psi) atomized diesel directly in the cylinder near the top of the compression cycle. The diesel instantly ignites as it comes out of the injector. This differs from a standard gasoline engine in that the spark plug ignites fuel/air mixture that is already in the cylinder**. Hydroxy has a better chance of speeding up the burn process of a gas engine due to the flame front starting at the plug and working its way to the piston in sort of a triangular shape. Diesel fuel can only ignite as fast as it comes out of the injector, which it already does.
In regards to the newer cummins having a map sensor and O2 sensor...you are correct, it does. However, keep in mind the throttling/injection methods. The cummins has those sensors for completely different reasons from a gas engine. A gas engine must run near stoich, and (unless it is mass air), relies on the map sensor to tell it the amount of air entering the engine to deliver the proper amount of fuel. Although the diesel is also doing this, it is strictly for emission purposes and operating the VGT turbo. I will touch on this a bit later, but altering your map/o2 signals on the cummins like you would a gas could possible improve your fuel economy, but will mostly screw with the emissions. Most importantly, if your truck has a catalyst, do not mess with the map/o2 sensor signals or you will destroy the cat, and you really don't want to know how much one of those costs to replace. It is also not nearly as easy to just gut/remove the cat on a newer diesel like a gas engine.
In regard to Bob's comment on diesel fuel economy...diesels are getting progressively worse fuel economy. Once again, this is due to emissions. I won't go into all of the detail as it doesn't matter, but just to give you an idea. New diesels (2010 model year), will be using urea. The exhaust runs through a diesel particulate filter to get rid of the solids (black smoke), then into a secondary catalyst to reduce nox and other emissions. In this 2nd catalyst, they inject urea to help remove the nox. Urea has similar chemicals to urine, and is a running joke in the industry. This diesel particulate filter (DPF) gets plugged up occasionally. To clean it out, the engine injects raw diesel into the catalyst raising its temperature significantly burning off these deposited. When this injector is functioning, you are getting nothing short of MPG down the drain. Simply put, the older diesels get significantly better fuel economy than their emissionized counterparts. This goes back way before the urea injection time frame to the first use of EGR.
In regards to the fuel economy of your step-dad's truck, there are a few things I can contribute to it. I have driven a few perkins diesels in my day in trucks and they were underpowered/undersized by all standards, an intentional thing. This is a perfect given to FE due to a smaller engine working near maximum capacity gets better economy than a larger engine working at partial capacity. I have an f350 as well, get right at 21mpg at about 550hp. When you consider the drop in engine size/power, the pattern is there. Although I do not know a whole lot about perkins engines, I do know that they are fairly advanced/efficient engines which plays a big role as well. I also bet that your dad's truck was geared properly for FE. My truck is geared for power/acceleration (since I race it) which totally kills any hope for good FE.
The whole inline 6 having more torque than a v8 is a myth that goes back as far as anyone alive today can remember. That is absolutely not true, although it does have merits in fact and I will cover a few of them here. This topic can get deep into engineering BS and I just don't care to go there. Inline 6 and v12 engines both have one very important quality...they are naturally balanced. Not all engines are naturally balanced, some worse than others. Inline 4's are horrible and often use balance shafts, v8's are pretty crappy which gives them that awesome "v8 rumble". I6 and v12 are perfectly balanced by nature. This lends themselves to a long engine life and smooth running. Their biggest drawbacks are length. It takes a lot more length to get a certain displacement out of 6 pistons straight in a line than side by side, and as thus, they are not a common passenger car engine.
Why does cummins (and many others) choose the I6 configuration? It is simply a very slight increase dependability. Does ford/international's v8 diesel undependable? Not at all. The powerstroke is a v8 as it makes it much easier to fit into a chassis, although interestingly enough, the ford chassis is big enough to fit the cummins in as well. Now, to finally answer the question, why does cummins 5.9l make torque at a lower rpm (since they both produce nearly the same with the powerstroke having more power on average for about 10 years) than the v8 ford? Simply put, it has to. Maximum allowable/safe rpm is governed by valve train (or even more simplified, valve spring pressure) and rotating assembly weight. It is exactly the same reason racers use lightened cranks, pistons, and rods...to lose rotational mass. With only having 6 pistons, the cummins version cannot spin the same rpm as a v8. The v8 has its rotating mass divided over more pistons (or more area). The v8 can spin faster in effect making more horsepower.
There is another reason which specially starts the myth and starts to get a little deep. At an even torque, the i6 must produce (and does in real life) more torque per cylinder than a v8. At a given rpm, there is a fixed number of power cycles for each engine, and the i6 has the CAPABILITY to produce more power per cylinder per cycle than a v8. Does this actually happen? Yes. It happens because the auto makers tune the v8s to make power up higher. Cummins made the v903 (15L) V8 engine to put into tanks. An i6 would not fit. Cummins also has a 15l inline 6. Comparing the highest powered models, the 903 put out 600 MORE horsepower than the i6 and 200ft lb more torque. How did it get so much extra horsepower? It is all in the RPM. They could turn that 903 cubic inch monster faster than they turn a cummins 5.9l, and it made more torque in the process. When it comes down to it, torque vs rpm is all in the tuning, and on a nearly consistent basis the v8 is tuned for RPM.
Since I specifically own internationals (and fords with international engines), work on and drive cummins engines, I will say with all of the seriousness that I can...a v8 is a good towing engine. They have totally different driving styles and someone who drives the other truck will not like it unless they are aware of the difference. They are drive completely different, but if you drive them like they are meant to be, they will both tow perfectly fine.
Diesel engines love to work and run their best near full power. I would not want to run a gas engine full throttle for very long as a personal opinion, but you can run a diesel full power nearly all its life without any problems. Truck drivers do this every day. Their trucks are generally speaking so underpowered that they have to hold it to the floor to go anywhere, which anyone who has got behind a slow accelerating truck can attest to. The engines are "small" for FE. I would almost guarantee that your dad's awesome FE is rested on this theory right here.
Do NOT add automatic trans fluid to your diesel. Does it work on the old engines? Well, yes, but with today's technology there are much better alternatives. PowerService is a very common item that I personally use, but there are many other, even better alternatives out there that you can research.
*The new 6.7l cummins and new ISX does have a throttle valve which works on conjunction with the VGT turbo to limit air under certain conditions for emissions only.
**There are direct injection gasoline engines in which the fuel injector is also directly in the cylinder. However, the spark plug ignites the fuel/air mixture in a very similar way as the conventional design. However, due to the direct injection, they are able to raise the compression ratios up to around 16:1, diesel levels, and extract nearly the same efficiency from the gas engine as a diesel.
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