Engine Torque Curves

I am a mechanical engineering student. This summer, I have been given a project to convert a boat from an IC gasoline engine to an electric motor (with appropriate speed controls). The current arrangement is an inboard/outboard configuration consisting of a Mercury Mercruiser MCM 120 hp engine (2.5 liters) and an Alpha 1 stern drive. My plan is to leave the drive (and associated prop and gearing) in place and directly couple the electric motor to it.

My problem is in selecting an electric motor. I cannot find power and torque curve information for the Mercury engine. Mercury considers it proprietary and will not share it. I know there are huge differences between what an electric motor's torque curve will look like compared to an IC engine and I do not wish to attempt to have them match. However, I would like to match (as closely as practical) the torque at a few points - 700 rpm (idle), 1000 rpm (cruise), and 1600 rpm (max that engine is used at).

It would be nice to place the engine on a dyno and measure this information for myself, but that is not an option. Does anyone out there happen to have this information (or can anyone direct me to it)? Or does anyone have any other ideas to help me over this bump? I suppose I can start from scratch and model the boat/system to come up with the required power/torque, but I'm not that confident in my ability and I am not sure where to start.

 

I took an engine to a local boat dealer and they had dyno facilities and checked the output of the engine. You may be able to get data for any similar engine and apply it for your purposes. Also if you consider max hp to be delivered at 5000 to 6000 rpm, you can calculate the torque required for that point.
Please check my work for me...not certified!
Power=work/second
work=(force)(distance)
distance=(radius)(angle in radians)
torque=(force)(radius)
therefore
power=(force)(radius)(angle in radians)/second
power=(torque)(angle in radians)/second

 

I think you can search to get a typical 2 stroke hp or torque curve as a function of rpm, so you might try using that curve and sub in your hp number and use the curve to interpolate the numbers at any rpm. This is based on my idea that most all 2stroke outboards seem to have max power at 5000 to 6000 rpm, so the power at other rpms should be proportional between different sizes of 2 strokes. .

 

sorry, Im talking 2 strokes, but all should apply. If the engine is just a standard otto cycle car type engine, I would search for automobile engine data and fit curves with that data.

 

I don't profess to know anything much about electric motors - but we won't let a little thing like ignorance get in the way of having my say!
I'd be very surprised if you could match the torque of any combustion engine with that of an electric one. Correct me if I'm worng (and I know someone on the board will! ) but an electric engine produces it's max torque at minimum revs - ie +/-1 rpm, whereas a combustion engine produces it's max torque much higher up the rev range - typically with a sterndrive that would be around 3000 rpm, with an outboard around 4000 rpm.

 

You are correct. An electric motor is going to produce its max torque at low rpms as opposed to the IC engine at higher rpms. That is why I don't have any intention of matching the entire curve. If that were even possible, the controls would be entirely too expensive. I just want to be able to produce similar power at particular points. I'm told that this particualr engine's max rpm range is something like 4400 - 4600. I believe that is where max horsepower should also occur with max torque being somewhere nearby. I am only interested in the torque up to 1600 rpm. The difference in efficiency between an electric motor and an IC engine is vast, so a much smaller motor (I'm guessing 35 - 40 hp) should meet my demands. The main problem is in deciding what type of motor (and I'm planning on a DC motor) - should I use shunt, series, compound, or separately excited? Each has its own torque vs speed characteristics. By knowing what the original Merc was doing at my selected points, I should be able to work a solution.

 

I guess I can be a bit more accurate on what my thoughts are on matching the electric motor to the IC engine. Becuase the electric motor has high torque at low rpms, the most important torque value to me is the one at my high end - 1600 rpms. By matching this point, through proper motor selection and drive control, I believe I can produce what I want at the lower end of my range. I feel like I am rambling on in each of my posts, but I really only understand enough to be dangerous and I am not sure I am explaining myself well.

 

Dale Calkins had an algorithmic fit to HP / RPM curves for marine engines in the 1983 (?) Transactions of SNAME.

 

I bet you will only be able to get displacement speeds unless you setup a geartrain to keep the motor in a suitable torque range.

 

The big problem with electrics of course is the low energy density of batteries as compared with fuel. I don't expect you'd want to plane. All those requirements for your design idea can and should be checked out before buying anything. Sounds like a lot of fun. Would love to be doing the project with you. Enjoy your student time.

 

I cant help myself. Suggestion: once you find a power requirement(Id do it from boat data curves which are available ) as a function of design speed, figure out how many batteries you will need to operate for say 10 minutes or 600 seconds. Power is work per second, and batteries all have a capacity which is a measure of stored energy or ability to do work(1 to 1). So you can quickly check feasibility by calculating how many batteries are reqd to run for 10 minutes at the power required for a certain speed. You will probably come to understand why only trolling motors are electric ( the purpose of this torture I expect!) or run from combustion powered electric generators ( or nuclear).

 

In other words, a way to approach the problem is to select a feasible power motor and boat design speed based on the battery requirements and gear /prop to that.

 

Also somewhere on the web, I ran across a site where someone was trying to create an electric powered Bolger Diablo. You might google for that and see what he did.

 

Here's the link:
http://www.psnw.com/~jmrudholm/etekoutboard.html

 

A small helpful excel sheet

The excel sheet I've joined may help you to recalculate the output of the engine. Engines have pretty standart outputs and the sheet is enough precise for your task.

Interesting project where you'll learn a lot. The batteries will cause you a lot of problems (wheight, etc) An alternative is a generator/batteries/electric engine as it's beginning to be done in ships and yachts. For a small boat is a very interesting project