Basic (stupid) questions about gas and diesel engines.

[CobyRupert]

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?

 

[Richard]

1: Yes, no clue
a) no idea, probably depends on what engine is using as excess goes back to tank. As a practical matter, when I 'treat' my two, I don't worry too much about that....as the treatment dissapates in the fuel tank, it will work its way through the supply tubes (or so my thinking goes)

2: Would seem to me that both would be inhaling the full displacement of air (forgetting about turbos)

Compression: My take is, the volume of the cylinder is for example, "10" when piston is at bottom of stroke. When piston is on compression stroke it takes that displacement of air, and compresses it down to say "1". There is now 1 "unit" of air in this chamber BUT, it's the same VOUME of air as when the chamber was full sized (or a 10) so now the compressed volume is under pressure....viola the 10:1 compression ratio.

I'd presume there is a more extravagant mathematical formula for this, if so, I'm unaware....just using chalk board math.

 

[MossRoad]

I though all diesels were fuel injected. And all engines, both gas and diesel, that have fuel injection have to have a return line to the fuel tank.

 

[bcp]

What's a diesel engine?

Only one that ignites injected diesel fuel by compression, or is a spark-ignition engine that is designed to run on diesel fuel a diesel, too?



Bruce

 

[mred2]

The return line, returns excess fuel to the tank. It is to maintain the correct fuel pressure in the main line. Not a good way to mix the fuel.
Compression on an engine is measured by taking the volume of the chamber when the piston is all the way down and comparing it to the volume of the chamber when the piston is all the way up. The numbers, one over the other, is the compression, given no compression loss. As things ware, rings, etc. the engine will lose compression, blow by, and power.

 

[i7win7]

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)?

No, gravity feed engines don't need it. A low mounted tank uses a lift pump to get fuel to the injection (high pressure) pump. The low pressure return helps to purge air from fuel lines. Truckers in the winter will run a high idle so the warmer return fuel helps to prevent fuel gelling.

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.

Diesels can have a throttle plate. A model airplane glow engine uses a glow plug to run. Removing the head and replacing it with a contra piston head (allows for an adjustable higher compression) model engine will run on kerosene. Early diesel tractors were started on gas, then switched to diesel fuel.

Diesel fuel is less flammable than gas. Compressing a large volume of air raises the temperature up to the flash point of the fuel. A glow plug is a coil of wire (tungsten?) that gets hot enough to glow and help ignite the next air/fuel charge. this device allows a lower compression level to be used, can be powered for cold weather starting.

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?

No
Compression is the mathematical ratio of cylinder volume with piston at the bottom vs the top.
Air/Fuel ratio: Gas engines 15-17x air to fuel, Diesel up to 70x air to fuel

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?

Sorta Yes, naturally aspired engine. Super charging and turbo charging increases the air volume inside cylinder, raising the compession.

 

[Hay Dude]

What's a diesel engine?

Only one that ignites injected diesel fuel by compression, or is a spark-ignition engine that is designed to run on diesel fuel a diesel, too?



Bruce

Some old Farmalls were both!

 

[Tx Jim]

Some old Farmalls were both!

Ditto some older IHC tractors have spark plugs on 1 side of cylinder head & diesel injectors on the other side. Fuel exiting fuel return line to tank is at low pressure(dribbling)

 

[LouNY]

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?

Yes, they all have a return line.
The pressure is a low pressure return to tank of excess fuel

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)?

They will return more fuel with a higher rpm while not loaded, high idles with no load.
It will vary depending on the engine but it is not real high on mechanically injected engines.
When I had mine disconnected on my Oliver 1550 and IH 574 at an idle it was just a good dribble,
with higher rpm and no load it was more, I never measured it when I was working on those engines.
But I wouldn't say it was much more then a qt. per minute.

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?

The gas engine at an idle with high vacuum will draw in less air then when it's loaded and the vacuum is decreased,
which is why the power valve in a carburetor opens with a decrease in vacuum to put in more fuel with the increased load resulting in less manifold vacuum.
The compression ratio is a math calculation of the volume differences at Top Dead Center compared to Bottom Dead Center.

The IH engines mentioned had a third valve in the head that was activated when you pulled the lever up to start on gas, once you got it started and the head warmed up you shoved the lever back down which took the decompression valve out of the engine operation and allowed the diesel to flow to the injectors.
When you started those engines they were on gas till you switched over for diesel.
The older engines that ran on kerosene or Distillate as it was called started on gas and when warmed up you switched it to distillate which came through the same carb as the gasoline. I heard many a story of the difficulties in getting one of those engines started if it got stalled or shut off while running on distillate. Haveing to drain the carb, pulling the plugs and cleaning and drying them to try and get it restarted on gas. Those engines were also very low compression, I believe around 7 to 1.

 

[finn1]

The later 7.3 liter Navistar Diesels don’t have a return line, nor do engines with common rail fuel systems.

Spark ignition gas engines run within a narrow air/ fuel ratio window, around 14.6:1, so they use a throttle plate to reduce airflow and increase vacuum in the cylinder at less than wot. Diesels run from around 15:1 to 100:1. The cylinder always get a full charge of air.

Direct injection gas engines can run leaner air fuel ratios than traditional carbureted or port injection engines.