I am not an experienced mechanic so this question might seem ridiculous. I got the impression when talking that a long stroke low rpm engine will get the crank journals out of round faster than a high rpm engine. I know that low rpm long stroke engines are known for longevity. What are the real pros and cons? I am just curious.
- Thread starter R Wyler
- Start date
typicaly a long stroke engine will develop more torque. so would be used in more of a stationary engine or tractor type setting with a constant load,or a heavy load. a higher rpm engine would be used say more in a auto with gears to multiply the torque, but would develop more hp at higher rpms.as for longevity, i would wager the lower speeded engine would be easier on bearings in the real world at least in most cases.
Dan in North Houston
Member
One of my old companies installed four new large natural gas compressors, driven by Waukeshaw engines. They were designed to run at 1500 rpm, but we found that we couldn't keep up with the maintenance at that speed - continous problems, some serious. Slowing them down to about 1250 rpm cut the maintenance problems down by about 75%. It was actually cheaper for us to run four of the units at the slower speed than to run three at the rated speed.
Russ from MN
Well-known Member
- Location
- Bemidji MN
If it has good oil pressure and good oil it shouldn't matter. Recently there was a thread about extremely slow idle speed and I think that could be hard on bearings.
As someone who has worked on quite a few of both types of engines all I can say is the low RPM types seem to run just as long, or longer than the hig RPM engines. As far as crank journal wear goes, as long as you use a good quality oil, and maintain it properly, wear should be negligable on either one, regardless of how long it has been run. Heck, I've seen some of the old engines ( Ie- 4 and 6 cylinder Murpys) that had been in service for 20 years and are still running great, and some that were worn to the point that you could literally move the crank up and down with a pry bar. Even with the worn crank the engine was still running, but the guy had switched to a mizture of engine oil and 90w gear oil to keep the pressure up until we could find him a replacement engine. By the same token I've seen high speed engines in daily service for 12 plus years with absolutely no crank, liner, or rod bearign wear, and some with half that amount of use that were worn completely out. In all cases proper maintenance, more than anything else, was the root cause of the wear, or lack thereof. In other words, take care of your engine and it will take care of you, don't, and it will cost you way more in the end than using quality lubricants, and filters, ever would.
Speaking as a retired railroad Machinist/Mechanic:
A General Motors,Electro Motive Division 16V645
(V16,645 cubic inch per cylinder)Locomotive 2
cycle engine idles at 200 RPM,top goverened speed
is 800 RPM, Stroke is 10 inch"s. Horsepower is around 3000, depending of if the engine is super-
charged or turbo charged. I am not aware of any
specific bearing problems. Back in the 1970"s, as
part of preventave maintanance we used to change
out the bottom connecting rod bearings. Only the
bottom, as, on a 2 cycle engine, there was power on the downstroke, and compression on the upstroke
so only the bottom bearing insert shell was
subject to any major wear. We seldom noticed any unusual wear,and it the late 70"s we quit doing
the changeout, as it wasn"t cost effective.
Locomotive main bearing and connecting rod
bearings were silver plated, and detection of
silver in the oil is a part of oil sample
analysis, now, as indication of bearing wear.
A General Motors,Electro Motive Division 16V645
(V16,645 cubic inch per cylinder)Locomotive 2
cycle engine idles at 200 RPM,top goverened speed
is 800 RPM, Stroke is 10 inch"s. Horsepower is around 3000, depending of if the engine is super-
charged or turbo charged. I am not aware of any
specific bearing problems. Back in the 1970"s, as
part of preventave maintanance we used to change
out the bottom connecting rod bearings. Only the
bottom, as, on a 2 cycle engine, there was power on the downstroke, and compression on the upstroke
so only the bottom bearing insert shell was
subject to any major wear. We seldom noticed any unusual wear,and it the late 70"s we quit doing
the changeout, as it wasn"t cost effective.
Locomotive main bearing and connecting rod
bearings were silver plated, and detection of
silver in the oil is a part of oil sample
analysis, now, as indication of bearing wear.
in-too-deep
Well-known Member
How do you start those things? Especially in winter. Locomotives fascinate me. Steam ones, too.
Bob M
Well-known Member
Depends on the type of locomotive traction generator - DC or AC.
DC (older) locomotives are cranked by dumping battery power to the main generator. The generator becomes a motor which spins up the diesel engine.
Newer AC locomotives require a separate geared electric or pneumatic starter. This is because the traction generator (actually an AC alternator) cannot be motored on battery current. Two starters are are generally used to achieve the crankshaft RPM required for reliable cold starting.
AFIK no starting aids (glow plugs, manifold heaters, etc) are required on mainline diesel locomotives.
Incidentally the sound of a pneumatic-start locomotive getting cranking up is one you are not likely to forget!
DC (older) locomotives are cranked by dumping battery power to the main generator. The generator becomes a motor which spins up the diesel engine.
Newer AC locomotives require a separate geared electric or pneumatic starter. This is because the traction generator (actually an AC alternator) cannot be motored on battery current. Two starters are are generally used to achieve the crankshaft RPM required for reliable cold starting.
AFIK no starting aids (glow plugs, manifold heaters, etc) are required on mainline diesel locomotives.
Incidentally the sound of a pneumatic-start locomotive getting cranking up is one you are not likely to forget!
buickanddeere
Well-known Member
Take the same cylinder heads, induction, exhaust cam etc from a low rpm, long stroke engine . And change the short block for a large bore, short stroke unit of IDENTICAL displacement.
HP and torque will be identical along with slower piston speed.
Why does a short stroke large bore engine get used in a high rpm application? The small bore engine does not have room for large valves and large dia ports to feed a engine making high HP.
The small bore engine's small ports they choke off high rpm flow.
The small bore engine since it can't breath at high rpms . It's equipped with a short duration cam which builds low end torque and further chokes off high rpm power. Compression is kept a little lower to limit compression pressure and detonation.
The large bore, short stroke engine with the larger ports and valves which can provide flow a higher rpms. The engine is cammed with an longer duration cam and higher compression . Which makes higher mid range torque and top end power.
So yes, no and maybe .
HP and torque will be identical along with slower piston speed.
Why does a short stroke large bore engine get used in a high rpm application? The small bore engine does not have room for large valves and large dia ports to feed a engine making high HP.
The small bore engine's small ports they choke off high rpm flow.
The small bore engine since it can't breath at high rpms . It's equipped with a short duration cam which builds low end torque and further chokes off high rpm power. Compression is kept a little lower to limit compression pressure and detonation.
The large bore, short stroke engine with the larger ports and valves which can provide flow a higher rpms. The engine is cammed with an longer duration cam and higher compression . Which makes higher mid range torque and top end power.
So yes, no and maybe .
todd hamilton
Member
Hadda '78 Oldsmobile Delta 88 2 door Brougham with a 403 long stroke...shecwas a smoooooth ride 
theoldtexasfarmer
Member
On JD utility tractors,3 and 4 cylinder engines,with 4.33 stroke,exept one with 5.0 in. stroke,all are rated at 2400 or 2500 rpm.All appear to be good engines and that the 5 in stroke engine works well at 2500 rpm.Could the 3 cylinder engine be engineered with a 5 in stroke and could it hold up at 2500 rpm?How much more hp could it make? This came to mind while working my 3 cylinder 5403 hard at 2400 rpm.A three cylinder turbo long stroke JD,what do you think?
buickanddeere
Well-known Member
As a general rule of thumb on old pre emission mechanical
pump engines .A natural aspirated diesel at 1800rpm will
make 1/4 HP per cubic inch displacement .
A turbo diesel at 1800rpm approx 1/3 HP per cu"
A turbo intercooled engine at best makes 1/2 HP per cu"
when pushed hard.
pump engines .A natural aspirated diesel at 1800rpm will
make 1/4 HP per cubic inch displacement .
A turbo diesel at 1800rpm approx 1/3 HP per cu"
A turbo intercooled engine at best makes 1/2 HP per cu"
when pushed hard.
BushogPapa
Well-known Member
The Short-Stoke engine is going to have side-load problems, if used at low RPM because of increased Crank/Rod angles..also, a Short-Stroke engine usually has much shorter pistons that cannot spread that side load over as much area..
For general "Field-Work" use, the increased cam timing of the high RPM engine would equal less work done on a given amount of fuel..
An engine's Brake Specific Fuel Consumption (BSFC) is always lowest at the Max Torque RPM..NOT at "Max HP RPM"..
Ron..
buickanddeere
Well-known Member
Concerns about side loading are more to do with rod to stroke ratios. A heavier piston causes more loading than a light piston.
The cam timing , head, induction exhaust if identical will make no difference on the long stroke/small bore vs short stroke/large bore.
Certainly operation at the rpms where peak volumetric efficiency occurs. Is the point where peak torque and peak thermal efficiency occurs.
The cam timing , head, induction exhaust if identical will make no difference on the long stroke/small bore vs short stroke/large bore.
Certainly operation at the rpms where peak volumetric efficiency occurs. Is the point where peak torque and peak thermal efficiency occurs.
Never heard one of the locomotive engines starting but I have heard a big, vertically opposed, Fairbanks Morse fire off with an air starter. We had two of then onboard my first ship, when I was in the Navy, and regardless of where you were onboard the high pitched whine of the starter and the rumble of the emergency generators was unmistakable. It was one of those sounds you hated to hear because it meant something was screwed up, but loved to hear at the same time.
buickanddeere
Well-known Member
Watch the movie Unstoppable. In an opening scene engine number 777 , they were thinking 666 but anyways. Engine 777 does a cold start while rattling snorting, hammering and coughs to life in a nasty cloud of smoke.
Intake runner length also affects power capability. Long runners will make more low-RPM torque and give up some high-RPM power. A short intake runner will make more top-end power and less bottom-end torque.
That's why Dodge and Toyota have butterfly valves in their big V8 intakes. They can shorten and lengthen the intake runners to help broaden the power curve.
That's why Dodge and Toyota have butterfly valves in their big V8 intakes. They can shorten and lengthen the intake runners to help broaden the power curve.
Indiana Ken
Well-known Member
To the above I would repeat - torque is not determined by stroke! Maximum torque occurs at the RPM (for any stroke) at which the cylinder, most completely fills with air/fuel mixture. The engine designer determines the RPM point by port sizing, valve sizing, cam timing etc.
To the above I would add - long stroke engines have a smaller bore size for a given displacement. This is useful for heavy duty engines as it makes for a shorter crankshaft; a shorter crankshaft is stronger. In addtion the smaller pistons generate less force reducing bearing loads for a given torque level.
In applications where higher horse power and lighter weight is desired a larger bore is typically used because it allows for operation at higher RPM as explained in buickanddeere's post above.
As far as engine speed goes; crankshaft RPM is well understood. However, there is also piston speed to consider; average piston speed is given by; Piston Speed (ft/minute) = 0.166 x Stroke (inches) x RPM. Some of the long stroke engines we like to think are slow speed are really moving on the inside.
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