How to Size an Engine

Hi, Chester23, & welcome.

This is out of my area, so I can't add much, but sounds like you've got a good project going there.

Can I suggest that you give some idea of what performance you're looking for, in terms of top speed, time to accelerate etc.? This would give the-folks-who-know more to work with. It would also mean you could compare these figures with those of other existing lightweight vehicles, so you could get an idea of whether you are in the right 'ballpark'.

Good luck,

John.

Well I would say I'm looking for something that will get me up to at least 55mph. And as far as acceleration goes I don't need to be a demon off the line but as long as it goes fast enough to not get run over then its all good. When I put the figures in the calculating program it said it would take 6.2 seconds to get me up to 62 mph, and that's plenty fast enough for me. But when I got those figures they almost sounded wrong because that's a lot of performance for a 26 horsepower engine. So I was just wondering if I did it wrong and if I did I was hoping someone here could re figure it out for me.

Chester23 -- There are countless thousands of automobile enthusiasts in the USA and a number of other countries that have built their own unique automobiles for a wide variety of purposes all the way from everyday transportation to specialized high performance and racing types. Most all do not attempt such a project until they have a fair amount of experience with driving and a good understanding of how motor vehicles work and how to fix them. For most the effort involves liberal use of parts from existing production cars and a minimum of special fabrication. As such efforts extend into racing cars there is less use of production car parts and more specially built parts.

Fortunately your project suggests using as many production car parts as possible. This will be the case for all the other stuff that comprises a car besides the engine , transmission, wheels and a frame. You need suspension, springs, shocks, steering gear, brakes, fuel tank, radiator, fluids like fuel, water and oil, exhaust system, electrical system with alternator, battery, wiring, instruments, something for the driver to sit on, control pedals, levers, cables, switches. Then you need things to protect the driver from the elements, road debris, flying insects. For the sake of efficiency at high speeds some streamlining body panels can be desirable. Then there are the various mandatory safety devices such as bumpers, seatbelts, windshield wipers, horn, lights and their various controls. And also items that add to the driver's comfort, upholstery, heater, air conditioning and ventilation hardware.

It's important to note here that many of the above mentioned items can be reduced in size and weight if a 2 or 3 wheeled motorcycle configuration is chosen instead of a 4 wheel automobile configuration. When you pick the motorcycle arrangement much of the safety equipment is worn by the driver rather than being a part of the vehicle.

Performance (in a straight line) at legal highway speeds is mostly a function of the power to weight ratio. When you calculate 6.2 seconds for 0-62mph you are about right for your 587 pounds divided by 26 hp = 22.6 lbs/hp. These numbers are pretty close to a 350 or 400 cc motorcycle.

Where you've made a mistake is in your estimation of the total vehicle weight. You've forgotten about a lot of the necessary items I've listed above. They add a surprising amount of weight. (ask anyone who has built a car for drag racing or closed track racing). From my experience a well designed one person automobile for general transportation with the power plant and drive train you suggest will weigh in the range of 1000 pounds with absolutely minimum physical comfort items for the driver and more like 1300-1400 pounds with full "creature comforts" Add about 100 pounds if you want to make it a two seater. This assumes you use well engineered components borrowed from sub compact or "mini" production vehicles of similar weight. I think I can safely say here that at your level of experience you will want to avoid attempts to make any of your own parts whose failure would produce any kind of safety hazard.

In summary I would say that you could expect performance comparable to an older 40 hp Volkswagen Beetle that weighs in the neighborhood of 1900 pounds (but has a fully enclosed weatherproof body and seating for 4 people.) I expect you would get superior highway mileage; but you may be disappointed by the performance and mileage "around town". That Kohler engine sounds like an industrial type that is designed for continuous duty at one speed and therefore may have design and performance compromises at higher or especially lower speeds. You may well be able to overcome the drivability issues with that hydrostatic drive setup; but at the expense of overall mechanical efficiency.

My advice to you would be to seek out a complete small vehicle like a little Fiat or some other European car into which you retrofit your engine and hydrostatic drive train. Best to choose an older rear engine or front engine/rear wheel drive type. Even in this case there will be a good bit of design and fabrication work to put it all together.

If you still want to build a complete vehicle I'd suggest an excellent reference book by Ron Champion, "Build Your Own Sports Car for As Little As L250" Haynes Publishing, Sparkford, Yeovil, Somerset, BA22 7JJ, UK and 861 Lawrence Drive, Newbury Park, CA 91320, USA www.haynes.co.uk

This book covers many details of frame construction and component selection for a car that is right in the same weight range as your project. It is written for the person of some mechanical ability and technical experience who has little or no experience in actually building an automobile from the frame up.

One more thought……. Although these small diesels are fairly heavy in weight for the amount of horsepower they produce you might still look at building a 3 wheeler or even a motor scooter type. The advantage in a three wheeler would come from the reduction in weight and drive train simplicity of a single rear drive wheel and two front steering wheels and the driver's seat in between. Also the three wheeler registered as a motorcycle would require fewer weight adding safety and protection features at the expense of somewhat greater exposure of the driver to the elements.

Ed Weldon

Thank you for your response, it's well written and you bring up many valid points. And although I'm going to disagree with many of the things you said none of it will be to disrespect you an any way. I don't want you to feel like I'm belittling any of your knowledge but I do think we are just looking at the project in a different way. Ha ha I hope that makes sense, but it may not cause I often have trouble explaining exactly what I mean.

Most all do not attempt such a project until they have a fair amount of experience with driving and a good understanding of how motor vehicles work and how to fix them.

This is a good point, I really do come into this with no prior knowledge of fabricating my own project. But being young at 23 and being very interested in such a project I think I have enough energy/stupidity to figure this out with the help of knowledgeable people and my own determination to accomplish something. I intend to look at the projects on this website to help me find out what I should use as far as brakes, suspension, steering, wheel setups, and electronics go. And as far as the hydrostatic drive system I intend to look at the projects on this site. When I get a general idea of what the other projects use I can hopefully come up with something close to what I would need for my own. And when I have that established then I will ask for the experts opinions to see what I got right and what I didn't and they can correct me. So doing it that way I think I can come up with a reasonably well put together vehicle even without much prior knowhow.

And also items that add to the driver's comfort, upholstery, heater, air conditioning and ventilation hardware.

In my attempts to make things as simple as possible I will not add any of these things which should make things much easier. But yes I will need most of the other things you listed.

It's important to note here that many of the above mentioned items can be reduced in size and weight if a 2 or 3 wheeled motorcycle configuration is chosen instead of a 4 wheel automobile configuration. When you pick the motorcycle arrangement much of the safety equipment is worn by the driver rather than being a part of the vehicle.

I thought about making a 3 wheeled setup because other people also mentioned it would need less work to build and also the law has softer restrictions on what I can and can't use. But owning a scooter right now I can't say I feel safe when riding, and that's my biggest reason to go with a four wheel vehicle, I would have more crumple zone around me and also more stability.

Performance (in a straight line) at legal highway speeds is mostly a function of the power to weight ratio. When you calculate 6.2 seconds for 0-62mph you are about right for your 587 pounds divided by 26 hp = 22.6 lbs/hp. These numbers are pretty close to a 350 or 400 cc motorcycle.

I'm glad that you looked this over for me cause at the moment I was stuck behind not knowing if 26 hp would be enough.

Where you've made a mistake is in your estimation of the total vehicle weight. You've forgotten about a lot of the necessary items I've listed above. They add a surprising amount of weight. (ask anyone who has built a car for drag racing or closed track racing). From my experience a well designed one person automobile for general transportation with the power plant and drive train you suggest will weigh in the range of 1000 pounds with absolutely minimum physical comfort items for the driver and more like 1300-1400 pounds with full "creature comforts" Add about 100 pounds if you want to make it a two seater. This assumes you use well engineered components borrowed from sub compact or "mini" production vehicles of similar weight. I think I can safely say here that at your level of experience you will want to avoid attempts to make any of your own parts whose failure would produce any kind of safety hazard.

I'll agree that Ive probably underestimated the weight of the vehicle but I wouldn't say by as much as you think. I plan on making this an extremely simple vehicle having only the absolute necessary items to make it run,legal,and safe,I don't even intend to in close the cab, everything will me open but a small windshield. At most I would say that it would weigh in at 1000 pounds, and even with that weight the calculator says it would take about 10 seconds to get up to 62, but yes I definitely need to get a more accurate measure of the weight before I proceed to build. Hopefully I wouldn't need to use parts from mini production vehicles cause they have a large line of parts for dune buggys that weight less because they are made for much smaller vehicles. And I plan on finding out on myself what needs to be fabricated but to have someone that has welding and metal working experience to do most of the fabricating.

That Kohler engine sounds like an industrial type that is designed for continuous duty at one speed and therefore may have design and performance compromises at higher or especially lower speeds. You may well be able to overcome the drivability issues with that hydrostatic drive setup; but at the expense of overall mechanical efficiency.

I believe the engine is ideally suited to work better at its max rpm but I think it is a variable rpm engine, I sure do need to look into that, if its not I know yanmar has variable rpm engines.

I will definitely take a look at that book you recommended, it looks like it would be full of things I need to know.

Chester -- I posted another answer before You posted your reply. So some of what I said will not well fit your needs. That's OK.

Now I have a better idea of what you are up to and would like to help. Please let us know what country you live in in order that we don't waste time with comments not useful to you. Much of what you do will be based on the tools, materials, parts and skills available to you. To the extent that we know about these things we can be more useful to you in discussing specifics.

I have had much experience with building hot rods and driving them, as well as amateur level racing cars. I'm also a retired mechanical engineer with a good range of fabrication skills and tools available to me. So that's where I'm coming from.

With respect to a couple of things you mentioned. I'd say ventilation is the most important and probably the easiest to do after you've fashioned a seat cushion. Nothing is more miserable to me than the heat coming out of leaky and uninsulated firewall and floorboards of a hot rod on a hot summer day. Combine that with a poor engine cooling system boiling over and a battery that won't turn the starter and you've got pure agony. That quickly gets old even for young "Turks" like yourself.

Another point on Yanmar diesels. They are first quality hardware and are a bit pricey. The small ones I know of are quite heavy for their rated power output being designed for use in deep keel type sailboats where weight is an advantage and absolute reliability is important. You may do well to broaden your search for small diesel engines.

It's very nice that you have access to someone with machinist and welding resources. Depending on how far that shop is from the place where you will be building your project you may find that developing in yourself some of the simpler capabilities of that shop may represent a worthwhile investment of your time and money. I started out with a small set of hand tools, a drill press, some welding equipment, a good vise mounted on a strong workbench and a supply of hacksaw blades and files. When I finally bought a 4x6" metal cutting bandsaw and an angle grinder I realized I had waited too long for those items.

Ed Weldon

you need select the engine based on application and has to know number of parameters like Torque characters of driven element of your creation,Duty cycle,No Hours of your engine driven creation etc i would suggest contact your nearest Kohler application engineer who would help you better an engine for any application cant be selected based on assumption and Thumb rules.Good luck

crm

Have a look at Go Cart engines. A 1000cc - 150 HP will send you screaming across the land if you can handle it. (Personally I would go for something smaller)

Why don't you try your skills out on a used go cart frame. Convert it to the diesel-hydrostatic drive system. Learn from that, and plan for modifications. Changing from the 8" go cart tires to 14" tires will change the vehicle dynamics dramatically. Good luck.

A design like the new Bombadier three wheeler using a water cooled motorcycle and with at least a 600 cc motor to enable you to git up and go. If your going to street it, this is the best way to go and NOT special construction. If your going to stay off public roads, the kohler will do the job. Crashed bikes are cheap and they have a title. I saw a small 3 wheel car buzzing around the other day and it's cool. For sale at the scooter store. Don't calculate too much. Are you running grease? is that the diesel reason?

Ray -- You're right on and practical with your suggestions. This is the best way for a newbie to get his feet wet and test his ideas. But if he really intends to drive his project everyday to work and in the rain, slush and traffic jams a closed car is the only real answer. Best to start with a pre-smog small import sedan, lighten it up as much as possible and work out the connection of the special CV transmission to a conventional drive shaft in a front motor/rear axle drive scheme. This might take some serious searching to find the right car.

An old rear engine import like a tiny Fiat is a possibility. Rotate the transaxle and use it's reverse gear function. Discard whatever other gears in the transmission you can to reduce friction drag. then the Kohler and CV tranny go where the back seat used to be. The fabrication challenge then becomes machining up an adapter plate with a support bearing to connect the CV transmission to the old bellhousing on the transaxle and then build a subframe out of square steel tubing to hold the engine and CV transmission. This whole setup then gets covered with an insulated aluminum housing, removable to access the engine for service. This may sound klugey, but I've seen several successful conversions of this type before with gasoline engines. One big issue is isolating the driver compartment from the engine heat and noise.

One of the neatest was a VW transporter (pickup version of the old microbus) with a mid engine set down low allowing a very functional pickup bed up above. Another very similar was a VW bus with a 425 cubic inch Olds V8 and appropriate frame, suspension and drivetrain. OK, not exactly what we're thinking of here; but it sure looked good.

Back in the 60' and 70's there were a number of VW dune buggy conversions in California that had rotated the entire engine/transaxle to a mid engine position. What they lost in off road traction capability they gained in on road cornering and handling if set up right.

Ed Weldon

Sorry I haven't replied to man of your posts, I come to realize from everyones suggestions that I need to take a long look at what I'm getting into before I do get into it.

Stop looking, and get your feet wet. Try something. The knowledge you learn from an afternoon of things that do not work, will teach you more than a week with a calculator and a book full of theories. Go for it.