Formulas for calculating the bore and stroke of a diesel engine

[peterl95124]

Minh-Thanh, those "perfect conditions" are good enough for calculating loads required of a Diesel con-rod, required injection pressure, and anything else you can think about. "flash point" and "auto ignition" temperatures are both irrelevant for Diesel operation. so what's your point ? Pete.

 

[minh-thanh]

Same old saying: never reach 700 degrees C !!!!

"flash point" and "auto ignition" temperatures are both irrelevant for Diesel operation. so what's your point ? Pete.

Ook...Ook....Ok....I stop here !!!!

 

[GreenTwin]

I think it is safe to say that when using a 21:1 compression ratio, diesel fuel will combust when injected into the cylinder at TDC.
So does it really matter what that exact temperature is ?
Regardless of the temperature, diesel will combust under these conditions, and so that is what is critical for a diesel engine to operate.

.

 

[ruben]

Hello thanks for your help

 

[ruben]

I think it is safe to say that when using a 21:1 compression ratio, diesel fuel will combust when injected into the cylinder at TDC.
So does it really matter what that exact temperature is ?
Regardless of the temperature, diesel will combust under these conditions, and so that is what is critical for a diesel engine to operate.

.

I don't care about the exact temperature, as long as the diesel burns, that's enough for me.

 

[Steamchick]

It becomes more complex when you are trying to make a million engines a year, to meet stringent emissions criteria, so you want to combust at a high temperature for (thermodynamic) efficiency but not exceed 900deg.C to avoid making toxic NOx gases in the exhaust. Collects a Gov't fine if you get it wrong! And complete combustion before the gases expand and cool below about 300C when CO combustion stops. - otherwise you get High CO and a Government fine.

But HOBBY engines are just that. Usually replicas of a real old engine, or a simple engineering principle (Oil burns better at high pressure and temperature). So fortunately we can still run them on proper old recipe oil fuel.
(Unlike Steam engine guys who now have to use smokeless coal and non-coal, etc. in various countries! Wood pellets in a boiler designed for Steam Coal are not good! And my 45 year old Moto Guzzi doesn't do well on today's 95octane 10% Ethanol/Petrol compared to the 101octane leaded stuff it was designed for.).
Enough of my whinging.
WELL DONE for making proper engines that work!
What a gorgeous engine in that video Green Twin.
Here's an engine i saw on Sunday at a show.

K2

 

[Steamchick]

Here's an engine I saw in Sunday at a show.

 

[xander janssen]

I'm also looking into building a diesel engine like the one of mr. Find Hansen. If you carefully follow his youtube videos, he gives away some numbers. His green engine has a bore of 20 mm and a stroke of 40 mm which compresses into a head clearance (diameter 20 mm) of 1 mm and a combustion chamber of diameter 7 mm and 8 mm deep. This for sure helps in mixing the air and the fuel as there will be a lot of swirling air. This is where I put my money on for my first design.

Indeed, (almost) adiabatic compression gives much higher pressure (due to increased temperature) than compression ratio would tell you. The numbers regarding forces are enormous as Mr. Hansen also clearly points out. Hence the need for some good lubrication and a very sturdy design.

Once my engine is running, I will make a full set of drawings (with at least the critical dimensions) and make them available to any other interested person.

 

[ajoeiam]

I'm also looking into building a diesel engine like the one of mr. Find Hansen. If you carefully follow his youtube videos, he gives away some numbers. His green engine has a bore of 20 mm and a stroke of 40 mm which compresses into a head clearance (diameter 20 mm) of 1 mm and a combustion chamber of diameter 7 mm and 8 mm deep. This for sure helps in mixing the air and the fuel as there will be a lot of swirling air. This is where I put my money on for my first design.

Indeed, (almost) adiabatic compression gives much higher pressure (due to increased temperature) than compression ratio would tell you. The numbers regarding forces are enormous as Mr. Hansen also clearly points out. Hence the need for some good lubrication and a very sturdy design.

Once my engine is running, I will make a full set of drawings (with at least the critical dimensions) and make them available to any other interested person.

Now that I'm looking forward to - - - lead on - - -

 

[ruben]

It becomes more complex when you are trying to make a million engines a year, to meet stringent emissions criteria, so you want to combust at a high temperature for (thermodynamic) efficiency but not exceed 900deg.C to avoid making toxic NOx gases in the exhaust. Collects a Gov't fine if you get it wrong! And complete combustion before the gases expand and cool below about 300C when CO combustion stops. - otherwise you get High CO and a Government fine.

But HOBBY engines are just that. Usually replicas of a real old engine, or a simple engineering principle (Oil burns better at high pressure and temperature). So fortunately we can still run them on proper old recipe oil fuel.
(Unlike Steam engine guys who now have to use smokeless coal and non-coal, etc. in various countries! Wood pellets in a boiler designed for Steam Coal are not good! And my 45 year old Moto Guzzi doesn't do well on today's 95octane 10% Ethanol/Petrol compared to the 101octane leaded stuff it was designed for.).
Enough of my whinging.
WELL DONE for making proper engines that work!
What a gorgeous engine in that video Green Twin.
Here's an engine i saw on Sunday at a show.
View attachment 159861
K2

Hi, I've been thinking about making that engine for a while now. To get the plans, do I have to buy it from this website or is there another way that they are free?

https://sites.google.com/site/fordengine1893

 

[ruben]

Hello Rubin,

It is reasonably easy to calculate the compression ratio using a pressure gauge.
Measure the max. pressure of the cylinder then divide by 14.7 atmospheric pressure
Example; cylinder pressure measured 80-PSI divide by 14.7 (Atmos. pressure), answer 5.4:1
Hope this is of assistance

Doug Baker
Scarborough WA

Yes thanks for the help

 

[ruben]

Now that I'm looking forward to the time to... - - move on... - -

I am also thinking of building a diesel engine like Mr. Find Hansen's. If you follow his YouTube videos closely, he will give you some figures. His eco-engine has a 20mm bore and 40mm stroke which is compressed into a cylinder head clearance (20mm bore) of 1mm and a combustion chamber of 7mm diameter and 8mm depth. This certainly helps with mixing the air and fuel as there will be a lot of swirling air. This is where I put my money down for my first design.In fact, (almost) adiabatic compression generates much higher pressure (due to the temperature rise) than the compression ratio indicates. The figures relating to the forces are huge, as Mr. Hansen also clearly points out. Hence the need for good lubrication and a very sturdy design.Once my engine is up and running, I will make a complete set of drawings (with at least the critical dimensions) and make them available to anyone else who is interested.

These measures can help a lot

 

[ruben]

I'm also looking into building a diesel engine like the one of mr. Find Hansen. If you carefully follow his youtube videos, he gives away some numbers. His green engine has a bore of 20 mm and a stroke of 40 mm which compresses into a head clearance (diameter 20 mm) of 1 mm and a combustion chamber of diameter 7 mm and 8 mm deep. This for sure helps in mixing the air and the fuel as there will be a lot of swirling air. This is where I put my money on for my first design.

Indeed, (almost) adiabatic compression gives much higher pressure (due to increased temperature) than compression ratio would tell you. The numbers regarding forces are enormous as Mr. Hansen also clearly points out. Hence the need for some good lubrication and a very sturdy design.

Once my engine is running, I will make a full set of drawings (with at least the critical dimensions) and make them available to any other interested person.

Yes i see the Channel

 

[ruben]

I'm also looking into building a diesel engine like the one of mr. Find Hansen. If you carefully follow his youtube videos, he gives away some numbers. His green engine has a bore of 20 mm and a stroke of 40 mm which compresses into a head clearance (diameter 20 mm) of 1 mm and a combustion chamber of diameter 7 mm and 8 mm deep. This for sure helps in mixing the air and the fuel as there will be a lot of swirling air. This is where I put my money on for my first design.

Indeed, (almost) adiabatic compression gives much higher pressure (due to increased temperature) than compression ratio would tell you. The numbers regarding forces are enormous as Mr. Hansen also clearly points out. Hence the need for some good lubrication and a very sturdy design.

Once my engine is running, I will make a full set of drawings (with at least the critical dimensions) and make them available to any other interested person.

For desing the cylinder cylinder head crankshaft and other

 

[Jens Eirik Skogstad]

Use this program to calculate adiabatic compression temperature to find how much temperature you get if the numbers is known.

https://calculator.academy/adiabatic-compression-temperature-calculator/

With long stroke give more heat by compression than a short stroke for same revolution in the engine due high speed of piston. Size of bore not important. It creates easier to start up the long stroke engine.

 

[ruben]

Use this program to calculate adiabatic compression temperature to find how much temperature you get if the numbers is known.

https://calculator.academy/adiabatic-compression-temperature-calculator/

With long stroke give more heat by compression than a short stroke for same revolution in the engine due high speed of piston. Size of bore not important. It creates easier to start up the long stroke engine.

Wow thanks for these idea

 

[Jens Eirik Skogstad]

Ruben..

In the Find Hansen diesel engine has compression ratio 20:1 while the truck diesel engine has compression ratio at 16:1 and the large ship engine has compression ratio at 12:1 depending on construction and calculated by adiabatic compression temperature. All these engines is direct injection diesel engine. Small diesel engine has more loss of heat than large diesel ship diesel engines hence we have high compression ratio in small diesel engine than large diesel engine.

Don't mix between direct injection and precamber diesel injection in same size 2 litre car diesel engine: Direct injection diesel engine 16:1 - 18:1 vs precamber injection diesel engine 20:1 - 25:1 due loss of heat in precamber injection is larger since the surface of precamber in cylinder head is large to lead the heat out of cylinder head into cooling system. Before the precamber diesel engine is started, then the glow plug inside the precamber is heated first before start up the engine.


Timing of injection is much earlier in the Find Hansen diesel than in the car diesel engine due lagg of ignition. Also it take time from the atomized fuel leave from injector until fuel drops ignites by heat of compression. Well atomized fuel ignites easier than coarse atomized fuel also much smaller drops of fuel is easier to heat up quickly until ignition begins.

 

[ruben]

And is there a way to know what diameter and stroke is needed for a specific pressure and temperature?

 

[GreenTwin]

A Detroit Diesel 671 engine has a 4.25" bore, and a 5" stroke, 18.7 : 1 compression ratio.
So the bore stroke ratio is 0.85 : 1.
Assuming this is a pretty typical bore/stroke ratio for a diesel, then just pick what you want your bore to be.

An example with a 2" bore, using the above ratio, then the stroke would be about 2.4".

I would use a 21:1 compression ratio for a model engine, at least as a starting point.
So it is just a matter of calculating the clearance required to get that 21 : 1 ratio.

The posts on the previous page said that the compression ratio is the ratio between the stroke and clearance, so the formula would be 2.4 = 21 * X
X is the clearance, and it would be 0.1142". (somebody check me on this)

This will not give you the specific pressure or temperature, but it should give an operational diesel engine, assuming everything else is functioning correctly.

.

 

[minh-thanh]

And is there a way to know what diameter and stroke is needed for a specific pressure and temperature?

My little experience with home made 10cc diesel engines
The compression ratio is good enough to run the engine on diesel is 18 -->20 : 1
But not enough
You have to make sure the tolerances of the piston, rings and cylinder are within a good enough range to achieve that compression ratio
You can't take their tolerances like 0.07, 0.08 and with a compression ratio of 20-1 and run it on diesel because then the air loss during compression will make the compression ratio of the engine no longer 20-1, but it will be like 14,15, 16 -1 , that will not be enough heat during the compression stroke to run on diesel
So what about tolerances of 0.02, 0.03 ? also not enough . You can not compare the compression ratio between 0.02, 0.03 tolerance with a poorly processed surface with 0.02, 0.03 tolerance and a smooth processed surface.
Compression ratio is a necessary condition but not sufficient
In addition, Temperature: If your cooling system is too efficient, it will affect the initial temperature to start the engine and maintain running
So what if the above conditions cannot be guaranteed?? There is still a way to run your engine by using a fuel or fuel mixture that is more combustible

So what about the actual air temperature during compression??
That's a difficult question that probably no one here can give you an exact number, Why??
First: including the factors that I mentioned above
Second: Lobe time - valve opening - closing time, does the valve time accurately reflect the lobe time? Because it depends on the spring, inertia ....bla bla.
- Air is not a fixed volume and it has "delay" (I don't know what to call it in English)
Suction port diameter, exhaust port, valve diameter ....
.....
Therefore, when the engine is in the suction process, it will not have enough volume of the engine
All of the above can only be determined and can only be measured with standard LAB rooms, with formulas with technical standards of mechanics ... hundreds of other things
Conclusion: For valve opening - closing time: somewhere near TDC and BDC