Fuel Injection Quantity for very small Diesel? 2

[PrematureWear]

Greetings all and thanks for reading my very first post!

I am developing a very small single-cylinder Diesel engine, looking for advice & tips...

It is a 1.84CI (30cc) two-stroke single cylinder engine of the opposed-piston type (no cyl. head, two crankshafts each driving one piston in each end of the cylinder barrel). Yes, I know it's little! The exhaust ports are 3/16" high giving an effective displacement of 1.38 CI (22.6cc). The engine will use an oil sump and have oil control rings South of the ports at TDC to prevent oil 'eating', a forced lube oil system, and an external air pump for charging.

I've done a fair amount of research and hmmming and hawwing and am still curious...

1)At max power, say around 16 or 18 air:fuel, what is an appropriate diesel fuel injection quantity? I really can't believe how difficult it's been getting a handle on this. Assume 100% V.E. for 22.6cc. Volume units for fuel 'shot size' are preferred, since I have to design & build the pump myself, which I can do, but need to know the approximate Max. injection quantity first!

2)What a good starting point for compression ratio? I know the thermo efficiency drops as cylinder size decreases, so it'll definitely be on the high side... Say around 24:1? I want to use commercial pump Diesel fuel.

True Diesels as small as 10cc have been successfully built (as seen in Strictly I.C. Magazine a few years back) and I figure 30cc is more than achievable, but I'm aiming for a fully developed high-class little powerplant.

Looking forward to educated & insightful replies... Any questions, just ask! And again thanks for reading!

Peace & Happy Discussing

PrematureWear

 

[JSteve2]

Wow that is small.

Assuming you are naturally aspirated, you are looking at around 0.077 mm^3 per stroke for stoichiometric operation, so you will want to back off a touch from there. Maybe 0.07 mm^3 will give you 16:1. These are back of the envelope numbers, but they are in the right ballpark. Finding a pump accurate at that point may be tough, but I'm not familiar with things in that range so maybe not. As far as mass goes, diesel is a little over 80% of water, so around 0.05 mg/stroke.

I have no experience with an engine so small, so I can't tell you about what compression ratio you should use.

 

[PrematureWear]

JSteve2,

Thanks much for your first reply!

Just out of curiosity, what #s are you using? I calculated 0.00013 cu. in. per shot at 15:1, with works out to 2.1 cu. mm--so I'm way over by your method. I used the density of diesel fuel of 7.09 lbs. per US gallon (32.58 cu. in. per pound) and I have 0.00006 pounds of air in the chamber at 22.6 cc/1.38 CI (at .075 pounds per cubic foot of air, or .00004 lbs. per cubic inch).

So at 15:1 I need 0.000004 pounds of diesel fuel. At the figured 32.58 CI per pound of fuel, that works out to 0.00013 cubic inches of fuel per shot, or 2.13 cubic mm--way over.

Where am I wrong?

Thanx in advance!

Peace to all,

PrematureWear

 

[PrematureWear]

Oh yeah, I almost forgot--I am designing and building the entire engine (including injection pump) completely from scratch, so obtaining said pump is no problem--provided I don't need a gazillionth of a cubic mm of fuel

Thanx & Peace,

PW

 

[JSteve2]

PW -
I'll go with your number. My original math is gone, but I did it from moles of air and consequent moles of fuel in SI units. However, in my BS check I forgot the numbers I'm used to are for multi-cylinder so I made a mistake somewhere and didn't catch it at the end (I was off by an order of magnitude in what I thought I saw in the BS check, but perhaps a shifted decimal is excusable).

Your numbers all check out (had to look up many of your units- #/in^3 ??) but your stated cylinder volume, cylinder air mass, resulting fuel mass, and final fuel volume look good. Your numbers square with my naturally aspirated engine experience on larger engines (once I remember how many cylinders they have!) so I think you've got it there.

 

[PrematureWear]

JSteve2,

Thanks for your confirmation, right now our design will go with these confirmed numbers...

Sorry about the pounds per cubic inch thing, all our reference books are old school

Again, thanks for forcing me to think

Peace

PW

 

[PJGD]

For what it is worth, the Tilling-Stevens TS-3 engine (3-cylinder 3.52 Liter opposed piston 2-stroke) in its lowest rating gave 117 BHP at 2,400 rev/min.

At rated speed, and at the efficiency shown on this graph, that works out to ~62 mm3/stroke on regular diesel fuel. Your engine will run at a [much] higher speed, but you may be able to scale your fuel delivery to piston area or similar to cross-check your calculations.

When it comes to the fuel injection, the 62 mm3/stroke would have been delivered over approx 25 crank degrees at a mean effective injection pressure of around 300 bar and a peak of ~450 bar. The single nozzle hole diameter was Ø0.55 mm with a nozzle opening pressure of ~200 bar.

PJGD

 

[Peter7307]

PW ,
I have to ask...what is the intended use for this engineering jewel?

Pete.

 

[PrematureWear]

PJGD,

Wow, thanks for the brochure! I'll definitely see if your info gives some more elightenment/confirmation.

BTW wouldn't I scale (approximately of course) the fuel delivery down to the single lung effective displacement, not just the piston area? (Vol. air per cylinder vs. fuel shot size per cyl.?)

Peter7307:

The intended use is the most important of all engineering applications: Personal entertainment

I don't know much about Diesels in practice but have always held a rather deep fondness for the operating principle... So I figured the best way to get real-world experience with one of these puppies was to roll my own.

Single cylinder engines have the highest potential for fuel economy for a given displacement, so one lung it is... except they shake like a paint mixer even as mild, low-compression SI mills. So imagine what a lightweight one-lung Diesel would be like! The opposed piston layout makes it possible to have a one cylinder engine that can be perfectly balanced, including twin identical counter-rotating flywheels to cancel intense rotational vibration from the enormous energy exchanges on a high-compression Diesel. She oughta be a real smoothie In addition, I can play with the phasing of the two crankshafts, allowing me larger transfer ports, and the ability to supercharge the living boogers out of 'er Not to mention fuel system development, cool little lube oil pressurization system pumps/regulators, nifty little crankshaft/conrod/piston assemblies and a funky scavenging pump/supercharger to boot!

Ultimately the engine will probably end up powering the workbench for quite some time while I admire my handiwork The original intention was to provide ultra-low fuel consumption, vibration-free power for a mountain bike using a little powerplant that simply specs out as extremely cool. I may end up running a high-efficiency alternator with it too, just for kicks... Guess I'm probably gonna have to build a couple

So, the intended application is A)creative expression, and B)have some serious fun!

Thanks for all your replies, and man does this site ever rock!

PW

 

[PJGD]

Yes, scale the fuel delivery to the cylinder displacement.

A rule of thumb for well developed naturally aspirated 4-stroke engines is that at rated speed they use ~1 mm3/stroke of fuel per 1 cubic inch displacement, hence a 1 liter cylinder will demand a ~60 mm3 injection. Also, according to Howarth (The Design of High Speed Diesel Engines), such an engine will be rated at 2,100 ft/min piston speed and produce 1.55 HP/in2 of piston area. One assumes that a 2-stroke would be something less than half these values.

In your case, you will not be able to do as good a job of matching the fuel injection to the combustion system so at the clean exhaust limit, your values will be lower still. Nevertheless, you will have fun and learn a lot with your exercise.

PJGD

 

[PrematureWear]

PJGD,

Curiosity makes me wonder why a 2-stroke will use less than half of your stated values... Wouldn't it scale down to approximately the effective displacement? (Total mech. disp. minus the portion of the stroke used by the exhaust ports?)
My effective disp. will be 1.38 CI or 22.6 cc.

I will use an external scavenging pump which will be slightly oversized, giving a cylinderful of air every time.

Still wondering about piston speed and max. realistic RPM with my tiny cylinder... The combustion time should be very short with such a small chamber. Also, my little mill doesn't have to last 10,000 hours

Thanks for the thoughts!

PW

 

[Peter7307]

PW,
Thanks for the reply and "Personal entertainment" must be best reason for doing what you are doing !

Good luck with it all.

Pete.

 

[PrematureWear]

Thanx pete... Holy smokes did this thread ever take off eh I'm really digging this site right now

PW

 

[j2bprometheus]

PrematureWear -

Since this engine is so small and is a two stroke, I am not certain what compression ratio should be used.

I have virtually no two stroke diesel experience.

Modern four stroke direct injection diesels for trucks typically have a compression ratio of about 17.

Indirect injection four stroke diesels typically have / had higher compression ratios ( 22- 24 to one)

The lower limit on compression ratio is set by startability and avoiding white smoke in upon start up in cold weather. The indirect injection engines have higher compression ratios since they have higher heat transfer losses and thus are harder to start.

Since your small engine is for hobby purposes, it need not start easily in cold weather.

On the hand, your engine is so small that the heat transfer losses must be large. This would suggest that a high compression ratio would be required.

Does anyone out there have compression ratios for some relatively small two stroke diesels?


j2bprometheus

 

[btrueblood]

Hmm. Kubota makes small diesels, but "small" has become relative in this thread. This engine is huge in comparison to the OP's proposal:

http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=2654&context=tractormuseumlit

It appears to be set up for 21:1 compression.

Wisconsion Robin aka Robin Engines also made small air- and water-cooled diesels, but I couldn't find any references for their CR in a quick google search.

 

[Nemesis4]

Mr. PW - I think "personal entertainment" is a perfectly good reason for doing something like your project. However, what is your main aim - interest in building the engine or results in running it? If it is mostly the running aspect, I have found from similar projects that (surprisingly) it is often cheaper and more convenient to use car-engine size components rather than model engine or small motorcycle size bits and also the results relate better to the "real world". For instance you could use two 1980's diesel Golf engines without their heads bolted block-face to block-face with the two cranks joined by a chain. The standard Golf injector pumps etc. could also be used. Additional advantages are that the lubricating systems, starting systems etc. are all there.
Anyhow, good luck with your project.

 

[elbowgreaser]

What a great site. I just joined. Pardon an EE asking a total-newbie question about the placard on my newly-acquired Detroit Diesel engine.

The placard says:

FUEL RATE AT ADV. HP 220.1 MM3/STROKE (several spaces, then)

ADV. HP 470 AT 2100 RPM

Question:

I'm assuming 220 mm3/intake stroke, correct?

Then I get approx. 1.32 liters/min at 2000 rpm (1000 4-stroke cycles for 6 cylinders).

This sounds high to me (about a gallon every 3 minutes), since a semi truck get about 5 miles/gallon on the road, and they sure don't go 5 miles in 3 minutes.

I'm just trying to get an idea of fuel flow (from first principles) at 2000 rpm under some sort of standard load condition.

Fuel flow at idle would be nice to know too.

I know these are rock bottom, boneheaad questions for the level of this thread, but I made a quick search in the faq and online without too much success or confidence in what I read...and I feel an answer from you guys would be more reliable anyway.

Thanks.

 

[MikeHalloran]

Elbowgreaser, please RF your message and start a thread of your own with an appropriate subject line.

Your numbers seem okay, if your DD is a four-stroke. Not all of them are.
OTR trucks don't run at full power all the time, only when accelerating uphill.
Your DD dealer should be able to bury you in technical information.




Mike Halloran
Pembroke Pines, FL, USA

 

[bentwings2]

Before you start machining take a look at this site. He has been around forever in the model airplane world. Many of his diesel conversions use a variable compression. The screw thing on top of the cylinder. You will note that he uses a barrel valve type carb as used on almost all glow fuel model motors. Very fine thread needle valves if you haven't seen them.

http://www.davisdieseldevelopment.com/home.php