What makes an engine efficient, and long lasting?

[BioHazard]

This is something I've been giving a lot of thought to lately. I'm not talking about a specific engine here. Let's say you were designing an engine from scratch, what types of things would give you both the most efficient, and the longest lasting engine? Or are those two different things?

For example, if you want 50 horsepower, you could get that with one giant cylinder or 28 small ones. How does the number of pistons effect the efficiency of the engine? Also, some pistons are sort of shaped like a beer can, while others are cut down really short. Does this reduce friction? Does it come at some other cost? What about the number of rings on the piston? Do less rings = less friction? I've seen 2 stroke engines with one ring...and diesels with 4 or 5.

Also, what about bore vs stroke? You can have two 1 liter engines with vastly different bore/stroke combinations. I know that longer stroke = low revving torque and short stroke is for high revving horsepower, but does it have any other effects?

What about compression ratios? Common knowledge says that higher CR means better efficiency and more horsepower. Does that mean it's most efficient to run any fuel at the highest compression rate possible without detonation? How does a high compression rate effect the overall longevity of the engine? Does it put more stress on the bearings? What about turbo chargers, how do they compare to simply using ultra high compression without one? Is a turbo going to reduce the life of the average engine?

Any other vague engine theory thoughts are welcome...

[mobile_bob]

i will take a stab at answering this

in no particular order

comp ratio for a diesel engine is generally no higher than is needed to provide good starting, at the lowest temperatures anticipated
burning the type of fuel you plan to use. higher compression ratio's than what is needed to accomplish this goal generally has little effect on
efficiency, but will have an affect on friction and stresses.

generally direct injection is a bit more efficient than indirect injection, although some of the idi engines are very good.

personally i like a longer stroke and smaller bore relationship,

larger bores are generally more efficient than smaller bores.

i like lots of piston rings, yes there is more friction, but the engine will be cleaner/less crankcase pressure in most cases with additional rings.

piston speeds below 1300 ft/min are generally thought of as long lived design goals

quality materials, such as induction hardened cylinder liners, generally wet sleeve engine's, using a thermostatically controlled cooling system
to maintain between 195 to as much as perhaps 205 degree's F

proper designed valve train components, good cam design to make the opening and closing events as low in stress as possible, most engine companies
have this optimized very well, although i am not certain the indians give this much thought at all.

alloy aluminum pistons, high silicon content seem to wear forever.

full flow pressurized lube system, xdrilled crank with full flow filtration and bypass filtration, provide for pressurized and well filtered oil to all wear components.

primary and secondary fuel filtration

excellent air filtration, generally paper elements of high quality surpass oil bath typical of indian engine's

matching the engine size/power to the anticipated load, in other words if you only need 5kw of power don't deploy an engine that is
rated at 20kw

some of the things that i look for first in a quality engine design.

probably forgot half of them just now also.

bob g

[Westcliffe01]

Good quality materials, appropriate for the task at hand
Good manufacturing processes and skilled workers in manufacturing and assembly
An effective quality system (parts are to print)
Engineers who know what they are doing and not just college grads
Attention to detail
Management who give the engineers enough latitude to design for long life (not always considered desirable or a high priority), the manufacturing people facilities that are kept up to date and well maintained and marketing people who do not "oversell" the product.
A workforce who takes pride in the product they produce and grasp the consequences of mistakes on the entire organization
Customers who are prepared to pay the additional amount for a proven product

[BioHazard]

We always talk about old engines as being designed to run very slow, 600 RPM or so, and modern engines are usually much faster - 1800 or more. What would it take to run a more modern engine at super low speeds? Obviously a big flywheel. Would a different cam design be more efficient than one designed for 1800 or so?

[Crofter]

A good analysis of the pro's and con's; First thing is your horsepower per pound engine weight ratio goes for a crap. Fuel efficiency goes down because the combustion event is prolonged so more loss of heat into the combustion chamber. If it is a gas engine combustion ratio has to be lowered because the piston escape velocity is so much slowed in relation to the flame propagation rate. (detonation) More loss of efficiency! May result in torsional resonance and crank breakage. Do a google on torsional resonance for an hours reading. Valve timing would have to be changed. Think of wing shooting birds. The faster they fly the more you need to lead them. 

I,ll let someone else do the benefits!

[wiebe]

We always talk about old engines as being designed to run very slow, 600 RPM or so, and modern engines are usually much faster - 1800 or more. What would it take to run a more modern engine at super low speeds? Obviously a big flywheel. Would a different cam design be more efficient than one designed for 1800 or so?

My thing] rpm ok ,but piston speed is as importend .
A engine with 10 ins stroke at 500rpm its piston is traveling just as fast as .
a engine with 5 ins stroke at 1000 rpm .

look at a big ships engine they must bow something about long lasting engine,s

[BioHazard]

So...theoretically...if you sped a 600 RPM lister up to 1800 RPM, if it didn't fly apart, would it be more efficient?

[mobile_bob]

it probably would be more efficient at higher speed,

when the speed gets below a certain point, as crofter alludes to, the time for heat loss is significantly longer than optimum


for the most part this is why a changfa type is more fuel efficient at 1800rpm than a lister/oid is a 650rpm.

bob g

[Carlb]

That may be so bob but the changfa doesn't sound as cool 

[BioHazard]

So...for another theoretical question...

The shorter the stroke of an engine, the faster it can spin while still keeping the piston speed low(er). Is that correct? Wouldn't that engine then be more efficient/longer lasting?


I always thought the slow engines were more efficient since they have more time to burn the fuel....

[Crofter]

When you shorten the stroke you must increase bore to keep displacement the same. Depending on engine size that can make it difficult to get good combustion chamber shape at the compression ratio desired.
In any case over square is biased more to high rpm. I thought you were bent low,

Spark ignition and compression ignition engines really have such different limitations in the fuel burning mechanics and chemistry that they are an apples and oranges comparison.

[deeiche]

rm /

[mike90045]

Any new research on ceramic piston/cylinder engines  ?

And I wonder what metal treatments would work well with listeroid bores (other than Listard)

[Westcliffe01]

High tech ceramics are frightfully expensive.  $15-20 for a piece the size of an insert for a cutting tool.  Its the same with carbide.  They are excruciatingly difficult to finish since the only material that will cut them is diamond.  One can make amazing stuff out of them, but entire engines - I don't know if the planet possesses enough of these minerals to even make that feasible.   Good quality cast iron is just fine and aluminum with cast iron liners is holding up well too.  But too soft or non uniform cast iron like from India and in the past from China is only good for a matter of hours.

Valves, valve seats, guides, valve train, cam working surfaces, crank surfaces,, all of these need careful material choices, surface treatment and hardening to an appropriate depth.   Modern injectors are either tool steel or have some carbide components inside.  Special magnetic steel if they are electronically actuated as opposed to being a "dumb nozzle".  Just yesterday I was at a small company that makes micro hole EDM machines for putting the nozzles onto injectors.  They hold a tolerance on their holes of 1um or 39 miilionths of an inch while doing this job day in and day out.

[BioHazard]

In any case over square is biased more to high rpm. I thought you were bent low,

Yeah, you guys really threw me for a loop with heat efficiency thing at speed. It looks like I just might end up with an 1800 RPM direct drive setup in the near future...

It seems like bob always has a way of ruining my plans.