Designing an engine

[Drei]

Hi,

I want to design an engine and i have some questions about bore volume compression ratio etc. I want to built a petrol engine and i also know and able to work some formulas regarding thermodynamics pressures temperatures etc. If you want to design an engine from where do you start, i was thinking from the torque or work right. Also can the rpm be increased in these small engines.
when you have determine the work you find the bore volume and the stroke needed to be able to compress that volume and achieve the right temperature to ignite the petrol.
I am not sure of this so any replies are appreciated

Drei

 

[mu38&Bg#]

What kind of engine are you designing? For what application(load)?

 

[wareagle]

There are many variables to consider. As dieslepilot asked, what is the engine's intended purpose? What is the targeted operational RPM? What is the power requirement of the engine (SHP, torque, etc.)? How long will the engine have to run during a work cycle? What will the engine use for fuel (already defined in this case)? What environment will the engine be operating in? Is there a physical size limitation? Weight limitations? What size machining equipment and tooling is available? Defining the above is a starting point for the engine's design.

The purpose of this forum is for model engines built for fun. Most of these models do not perform any work other than sustaining themselves for a run, so performance requirements are usually not a primary concern. If they do perform some work, usually it isn't a substantial load for the engine, and it likely is in short durations.

That being said, many folks here are certainly very capable of helping and will be willing to do so, but you might want to seek another source of information if your engine doesn't fit the model engine category. For a purpose built engine, a proper design is very important for the project's success.

If the engine does fit the model category, then you have found what is in my opinion the most knowledgable and friendly online community in the model engine hobby.

 

[Layne]

Designing "an engine" is easy, you can whip one up out of scrap bits without ever making a drawing. It probably won't be very powerful, or light, or last very long. If your engine needs to meet any specific need (particularly low weight or high rpm) it gets more complicated. Most of the engines here just need to look and sound cool, which is totally subject to the builders preference. If you need a certain power per displacement or power per weight, it can take a lot of engineering. Auto makers have entire teams of engineers working on their engines at great length (and most of them are still not very good).

 

[Drei]

I was thinking of powering a model power boat or something like it. I also tought of building a v twin, one of the reasons is taht till now i have only built on cylinder engins and the other reason it would look cool on the boat with certinly alott of torque. Till know i have only built 2 engins with one cylinder, the first one being the Nemett engine an the second is with the same specs of the Nemett but insted of OHC i designed it with the valve train system. I only desinged the new parts and machiend the other parts as per original drawings. I also have the drawings available if someone is intrested and also some photos of the machiening which i want to upload .
As for the new design i wanted to use almost the Nemett engine specs and materials but maybe if possible increase a bit more the rpm. The main reason for more rpm is beacause for a fast model power boat you need alott of rpm wright?
As for load, i dont thing that if i use a v twin with almost the same cc of the nemett engine (which is 14.7; and if its a v twin is 29.4) it will have more that the needed power and torque. As for the duration, i know a bit about the limintations of a home built model engine thus i would take care of it.

Drei

 

[gbritnell]

One thing to consider for use in a model boat would be that the engine should be water cooled. Most of these little engines get quite hot and with the engine in a confined space it would most likely get even hotter. The engine should have some type of water jacket at least around the cylinder area. My avatar shows a picture of a v-twin that I designed and built and it gets very hot after 2 minutes of running.
gbritnell

 

[Lakc]

This pair of books are a great place to start.
http://www.amazon.com/dp/0262700263/?tag=skimlinks_replacement-20
Im told there is a newer reference then this, but I used to have access to both volumes at work, so this is the one I know.

 

[Admiral_dk]

I'm not in anyway, trying to tell you not to build a multicylinder, but if max torque is your main concern, up to 400 cm3 (and probably bigger than 700 cm3) a single will make more torque than a twin (or more cylinders) all other being equal. If max HP is your goal, then about 125 cm3 per cylinder on a two-stroke is the optimum dividing factor - more for a four-stroke.

So power in any form will never be a reason for making multicylinder model engines. That said, I'm sure I'll make some when my future workshop is operational

 

[mu38&Bg#]

Designing a good cam will be important. Most model four stroke boat engine have a water jacket just below the head on the cylinder.

I can help with basic power absorption curves for boat props, but am not familiar with how fast a boat might be with a given prop-power setup. I was looking at some figures for converting an OS FS-30 four stroke for boat use and it seemed like it would be easy enough to go 30MPH with .5 HP at 11kRPM. I just don't know how much hull design and weight comes into play.

Typical model airplane four stroke engines operate 9-10,000RPM maximum for 1.25-1.5HP output. Most engine in this size do not run much faster, but have never been designed to do so. OS offered car versions of the FS-26 (.26ci) and the FS-40 (.40ci) that were rated at peak power at 17kRPM and up to 22kRPM range. I know the 26 with modification for power could run up to 26kRPM. The main difference was stronger valve springs to prevent valve float and a different camshaft to allow for efficient breathing at this RPM. I think it would be possible to increase RPM of a 15CC cylinder to 15k or more, but you must pay attention to piston weight and strength.

According to some info on modelenginenews.org the Nemett makes little power, maybe .7HP at 7400 RPM. The valves look small. There is a lot of room for improvement in this case. Increasing valve sizes and improving the cam would be a good place to begin.

Greg

 

[Drei]

Thanks Greg and all of you, i have more points to expalin. As for the cooling i was thinking of protruding the two cylinders above the boat deck but cooling isnt a problem because if it get very hot i will machien a water cylinder jacket like the nemett on simple and the only thing you have to change is the jacket.
As for the rpm i was thinking that if i dont achieve the desired rpm i would speed up the shaft by instaling a small 'gear box' with a greater ratio.

The main reason for building the new twin engine is for a good v twin challange also a challange for my own design. Then if the engine will first work and look good enough to fit a boat i will mount it on the boat.

As for the rpm nemett engine runs from 1000-6000 but if i achieve and 8k or 9k rpm i will be more than happy. My plan is to leave the same specs and modefy the cam lobes (more valve lift and duration), lighter piston and connecting rod and valve springs like Greg said.

I have anoter question, if i increase the compression ratio will it make any difference on rpm or torque or it will effect the fuel ignition point and wount start. I heard thet if you increase the comp ratio the octane ratio of the fuel has to be increased but i m not sure about it.

Drei

 

[mu38&Bg#]

Increasing compression ratio should increase torque output. Detonation in an engine this size shouldn't really be a problem with spark ignition. Ignition timing should be checked at operating load and temperature mainly for detonation, but it shouldn't be a problem. By the looks of the parts in the Nemett, in photos I've seen, I'd strengthen the rod and use a larger diameter wrist pin. The OS FS-91 has a 6mm wrist pin. The piston weighs 9.3g.

 

[Drei]

How can i design and machien a cam saft. In the Nemett engine the cam machiening is very easy and has very good results. Is there any programm that gives you the values of the lift and cut. If you somebody has or made the engine knows how is the data sheet.
If i determin the lift and duration of the cam from the engine cycle and stroke is there any programm or method which i can machien the lobe???

Thanks for the help
drei

 

[Maryak]

Drei,

A table of offsets in excel derived from a CAD profile of the cam. Then milled and dressed. Not the only way but a search here will show you others.

Hope this helps

Best Regards
Bob

 

[mu38&Bg#]

I posted the cam reply in another topic by mistake.

For cams, the hardest part is the design of a good cam. Making it, once you have the profile, is a math exercise best done with a spreadsheet. Intricate cam design is somewhat a secretive thing. There are no free programs for complex engine cam design, plenty of money is to be made by those that know what they're doing. Basic cam design software is readily available including this http://modelenginenews.org/design/index.html

Greg

Thanks Greg for the info, and yes thats where i saw the piston head holes which i think im going to drill some in my next engine.
As for the cam design, i used and tried the cam programm that you ave me on modelenginenews site. To have a high rpm engine does the accelertaion make any difference. I mean that the accleration is calculated by the programm so what does it mean if it come a high value or low what difference does it make more rpm or just high rate of accleration????

Thanks
Drei

Acceleration refers to motion of the valve train components. When it becomes excessive the valves bounce rather than follow the cam. Valve trains are complex when you are trying to get the most out of them. You'll want to do some research on cam design. Cam design isn't even covered in engine design books it's a topic in and of itself.

 

[Drei]

Yesterday i searched about some racing cam profiles and timing etc. I know Greg that on this forum and on the CAMcal you gave me doesnt regard and consider high performance. But sitt i will try in designing something like it and experiment with different timing and profiles. The link is http://www.circletrack.com/techarticles/ctrp_0701_camshaft_design_science/index.html
It has some good information even regarding valves overlap etc.. Then i inserted som values on the CAMcal which gave me some intresting cam profiles that im going to try .

 

[gbritnell]

Hi Drei,
Over the years there has been quite a lot of talk about cam timing, lobe overlap, machining etc. In a nutshell for model engine work I have found that moderate cam timing and overlap will produce good results. If you have followed some of the build threads on constructing I.C. engines you will find that there are enough problems to overcome when building these things without making a radical camshaft design.
Say someone builds an engine and they make a camshaft for it to a particular design. Now they build another cam with more radical valve events. How is a person supposed to know what it has done for the engine. By that I mean how does it change the torque and horsepower curve? Without some type of measuring device (dyno) about the only thing that can be ascertained is does it start easier or harder and how has the total rpm changed.
gbritnell