Basic (stupid) questions about gas and diesel engines.

[Bentrim]

My I insert a few thoughts? Diesel engines can only work when fuel is injected into the cylinder, it will ignite the fuel due to the heat generated by compression. Whether it is direct or indirect injection the liquid fuel is atomized at injection.

Often wondered why when working on engines a turbocharged engine had a larger combustion chamber than a natural aspirated engine (354 Perkins). Came to find there is a difference in static and dynamic compression ratio. Seems that the compression ratio is figured with the engine under boost. That is why an engine designed as a na engine is easier to start when cold.

As for the return lines some are designed as air elimination and leak off lines, but there are some engines that do not use any from the injectors (3208 cat for one) Note most diesel farm tractors use a return line from the fuel filters, it is a metered return which leaves air return to the tank.

Most diesels do not have a throttle plate but some do, many years ago working on a Nuffield tractor found it had a throttle plate and noticed the service manual had an adjustment procedure. Also be aware that most if not all Detroit diesel two strokes have an emergency flap to shut the engine down in case of a run away.

Was not aware but I understand that there are direct injected gasoline engines being produced now. Gasoline engines either had a throttle body injection or injectors that were in the intake manifold runners.

Yes when diesel in tractors were started be used manufacturers tried many different was to start them as battery capacity was very low back then. And IH used an engine that was both gas and diesel. The gas side engine had a small set carburetor and a regular ignition system, it was used for starting to get heat into the engine so it could run on diesel. Once started a lever was moved closing the "gas" portion of the cylinder head and the engine operated on diesel.

Now also remember back in the 30's and 40's there were tractors built as All Fuel. They started on gasoline and were then switched over to "tractor fuel" essentially kerosene or similar as they were called heavy fuel, but that said diesel fuel was not one of them. They also required a lower compression than one that was operated on gasoline. Have heard the fuel at then time was just a few cents a gallon when gasoline was 10 cents or more. The horsepower also was less in the all fuel tractors.

 

[number two]

My 2 stroke has injectors and a supercharger it's a little 4-43 screaming jimmy Detroit Diesel

I'm sure you mean 4-53?
Detroit's are cool.
Had 2 6V92TT's and 4 8V92TT's a lifetime ago.
Fun times.
Always love the sound of the GM diesels in the Higgins Landing Craft in the WW2 movies!

 

[LouNY]

yes, 4-53
If you work her hard for long ear plugs and muffs are nice.

 

[Mark @ Everlast]

Early tractors could run on gasoline (leaded heavily), tractor fuel ( a cheaper special blend of fuel oil no longer available), Kerosene (similar to tractor fuel), and in a very few cases diesel. In most cases of the tractors I have run, they were all cranked on gas, warmed up and then switched over to the fuel oil. In the cases like old caterpillar dozers that I've had the pleasure of running years ago, the spark plugs ceased output and switched over to the injection point. There was a half-way lever that allowed both to work until the transition completed. But once cranked it ran entirely like a normal diesel. Then, of course, I've run the old pony motors which used a side mounted 2 cylinder gas engine to crank over the diesel engine on compression release until it decided it was happy and ready to fire. Noisy and time consuming, but I guarantee you you'd never run out of batteries and could get your oil pressure built up before cranking.

Then there were the cases like the old John deeres which had two tanks, a smaller and a larger tank, and had to be cranked over and started on gas (small tank) and once warmed up could be switched over to run on fuel oil or kerosene. The problem is, that this wasn' t a true diesel engine. The compression was low enough on these engines that it could burn anything. The carb jets were open enough to allow it to pass through. There was no injection pump.

Diesel injectors have a return line that operates on the other side of the injector. I mean its cycles unused fuel after the pump "jerks" and back to the return port and back into the tank. They do not run under the same pressure when the injector "pops off". This is a low pressure line and is usually rubber. The pop off pressure of injectors is high, but can range. Usually in my experience, it is around 1200 to 1500 psi. Lower than that, you won't get good atomization which is required in a diesel to burn clean. At least this is for mechanical injected engines. Later diesels all have the return line from the injector. It's a small amount of fuel really. It should take a few hours to recycle the entire content of the tank. I mean a good medium sized diesel in a utility tractor burns 2 to 4 gallons an hour. A large one on a tractor can burn 10 to 15 gallons an hour. The return amount wouldn't be more than this rate I would think.

Compression ratios on older true Diesels start around 16:1 and can go over 30:1. The higher the compression, the noisier things get. Let's just say that cylinder compression readings are double gas burners at minimum and read between 300 and 500 psi, at least on the ones I've worked on.

As far as a vacuum, this is a misunderstanding. There is no venturi effect due to the design of a carburetor vs. the open intakes of the diesel, but there is vacuum. There is a negative pressure on a non turbo/super charged diesel engine in the manifold. Air just isn't flowing past a restrictive point like it does in a carb throttle and venturi area. This is why most older diesels in a car also run a vacuum pump to create the kind of negative pressure required to operate vacuum motors. Of course, automakers could have made special electrically operated motors and devices such as locking devices just for diesels (and a few did) but mostly they were too cheap to make a special different car other than the engine/transmission/drive train.

 

[Little Red Tractor]

Miscellaneous comments:
Mix your fuel, either before it goes into the tank or in the tank--whether you're adding a treatment, or kero to your diesel fuel. Let it run 10 minutes or so at low speed to have it saturate the whole system.

I've heard the same story about restsarting the engines on distillate. It was a pretty trashy fuel; but farmers who had natural gas wells had a ready supply.

 

[Gregster613]

Mark, You beat me to it and in a much much more better explanation. I was going to mention the old style pony motors that were used. Dad had a D7 that always was started with a pony motor. As long as the pony would start/run, the D7 would always start.

 

[SmallChange]

Some diesels recirculate a variable amount of exhaust into the intake. The idea is to have less oxygen available for some conditions -- I think it reduces how much NOx pollution you get. When you need more power, more oxygen, it won't do this, or not as much. In any case, even if there's no throttle plate and the engine is running slow enough it can aspirate all it wants, it may still typically not be true that it has full flow of fresh cold air.

 

[etpm]

I read about and saw pictures of a kit for gasoline tractors so they could run kerosene, diesel fuel, or any similar fuel oil. I also saw pictures of a DIY setup that did the same thing. Basically, and the DIY setup was pretty basic, a copper tube was wound around the exhaust manifold. This tube was connected to the fuel oil tank at one end and to a valve at the other. The engine was started on gasoline and warmed up. This also warmed the fuel oil in the copper tube. Once everything was good and not a valve was turned that directed the flow of hot fuel oil to the carb. The flow of gasoline was stopped at the same time. As I recall the valve controlled both the gasoline and fuel oil flow on the kit but the DIY setup required the farmer to turn two valves so the gasoline was slowly turned off and the fuel oil was turned on. This process was done gradually to prevent the tractor from stalling. I guess the farmer had to develop a "ffeel for when and how to make the changeover.
Eric

 

[Agvg]

Have had two tractors that was factory made to run on lamp oil or paraffin, one Ferguson TE-D and a Fordson Major, you looses power and the consumption goes up when running on alternative fuel.

 

[Anakist]

Was thinking about the differences between gas and diesel.

1) Do all diesels have a return line to fuel tank? Is this line to tank at the same pressure that injectors get?
a) Approximately how long would it take tractor to “cycle” (let’s say) 10 gallons through this recirculating loop. (Example, you’re using this cycling to filter fuel or mix fuel additives added to tank)?

2) Diesels have no throttle plate on intake. My understanding is that diesels will inhale the full cubic displacement of air every cycle. That all this cool air is why an idling diesel doesn’t warm up very fast as you’re not adding much fuel.
a) What about a gas engine? Let’s say gas engine has a 10:1 compression. With a (mostly) closed throttle plate, does the idling engine draw in the full cubic displacement of the piston and stroke, but because it’s at a vacuum, I think this means the actual number of air atoms sucked in by this displacement is less.
So what is the compression? Is the 10:1 compression inside cylinder, not 10x atmospheric pressure, but 10x whatever the “less than atmospheric pressure is” (vacuum) that was as created by the throttle plate? So it might only be (say) 6x atmospheric pressure? What are the real numbers?

Or is the air being pulled into cylinders always under a vacuum (because of pathway restrictions) so none of this matters very much when it comes to compression?

1: Not guaranteed but a lot do. Bleeding a diesel at the injectors is used when they don't have a return from there. I am super glad my Chinese wheel loader has one, it is terribly laid out so being able to manually pump until there is fuel coming back from that line is very much appreciated. The "pressure" in the return line should basically be 0.

a: It would take a while. I did a quick google on diesel injector return rates and there are test kits that come with tiny little bottles for catching returning fuel. Looks like single millilitres of leakage per 15 second test duration per injector. 10 gal is around 38 000 millilitres so a long time.

2: Look up volumetric efficiency (https://en.wikipedia.org/wiki/Volumetric_efficiency). It is the ratio of the swept capacity of the motor vs. how much air actually goes in. Swept capacity is the difference in volume between the space in the cylinder at bottom of stroke, and volume in the cylinder at the top of the stroke. You change this by changing the crank stroke or head height (for lack of a better term). Long stroke cranks get a bigger difference, short stroke cranks get a smaller difference. A head gasket change or decking the head makes a bigger relative difference at the top of stroke. Having the piston closer by 0.050" makes a bigger difference at 0.2" height, than at 2".

So swept capacity x volumetric efficiency will give you how much air is sucked into the combustion chamber each revolution. A very rough guide I have seen for gas engines is 85% volumetric efficiency. But that is very rough. There will be a graph and every engine will be different (https://www.onallcylinders.com/wp-content/uploads/2017/05/10/Presentation1.jpg). And if you change to a pod air filter, or change your exhaust, or get a new cam, or drive up to Denver, or make any change that modifies how air gets into the combustion chamber (even changing from standard to iridium spark plugs since the electrode is smaller) it will affect the volumetric efficiency.

I have seen old cars with a vacuum gauge as an indicator of how economically you are driving. A smaller throttle opening means less air and more vacuum, which means less fuel is needed. If you have forced induction you can have more than 1 volumetric efficiency.

James