Lehman Idle Speed

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Oct 6, 2007
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1983 42' Present Sundeck
I noticed a vibration this weekend at an idle on the Patricia Louise II and also realized that my port engine idle speed had dropped from about 700 RPM's to 600 RPM's. When I increased the port idle speed the vibration went away. The stbd engine idles at 850 RPM's. I suspect this is about right for a Lehman 135. I know I can't trust old analog Tach's to set this. Does Grainger offer a type of phototach to use where I can mark and shoot the front pulley?
While we have FL120s, not FL135s, an idle rpm of 850 seems a high to me. The idle speed of the FL120 is supposed to be about 600 rpm. The higher the idle rpm the "harder" the transmisisons will shift into gear, which could mean problems sooner rather than later. I don't know what kind of transmissions you have, but the maximum shifing rpm for the BW Velvet Drive is 1,000 rpm but that is considered much too high for long service. If your engines vibrate at a normal idle speed of 600-700 rpm, this may be an indication of a problem--- bad spray pattern from an injector, uneven fuel pressure to the injectors, etc.
Amazon lists several phototachs in their automotive section for under $100.* I've been meaning to get one (as well as an infrared thermometer), but the round tuits haven't come my way lately.* It's pretty simple technology, so these inexpensive units should work just fine (plus Amazon has a pretty good return policy).

I can get both engines synched within a few RPM by sound, and the four tachs (two on the FB and two in the cabin) read four totally different things, so all of the boat tachs are suspect.

I'd give one of the Amazon ones a try.
"Temp to high could be a lean burn (clogged)*to cold could be a rich burn*(leaking)."

That*is a gasoline engine model and*does not apply to a diesel engine. As a matter of fact, exactly the opposite is true.

One other thing to consider is the interaction between the two engines. You say when the one engine dropped to a 600 rpm idle while the other one remained at 850 you got vibration. You may have already done all of this, but (1) is it for sure the 600 rpm engine that is vibrating? (2) If you idle the 600 rpm engine with the other engine shut down, is the vibration still present? (3) if you idle the 850 rpm engine with the other engine shut down is the vibration still present?

Based on our experience with our old GB, there can be all sorts of vibrations in a boat from the engines (and even the props) interacting with each other in terms of vibration and vibration frequency. One engine running by itself might produce little to no vibration in the boat, but as soon as you start the second one, even when they are running in sync they can still generate "boat vibrations.'

For example if we are at normal cruise rpm on both main engines and we start the generator for some reason the boat acquires several new vibrations that aren't there if we're running the mains only or the generator only. But with all of them running there are little shudders and shakes that are the result of the harmonics or whatever of all three engines vibrating in their own unique ways. On a 35 year-old boat with equally old engines and generator it would not be worth the time, effort, or money to try to track these vibrations down and stop them. It's just "the way it is." Ride a Washington State ferry or even a big new container ship and they have all sorts of shudders and shakes. It's just the way it is.
"... if the primary was a 2 or 10 you could get a lean mixture and burn a piston."

IF you reduce the fuel supply to a diesel it will produce less power, run slower, then if you reduce it enough the engine will quit. As it does so the exhaust gas temperature just gets lower and lower.

Turbocharged diesels aside, there is no throttle and the same amount of air is in the cylinder regardless of load or speed, only the amount of fuel varies between idle and full power. As more fuel is burned the exhaust gas temperature increases, less fuel burned means a cooler exhaust. There is not really a mixture issue with a diesel until you get to the "smoke point" which is the point where there is insufficient air to completely combust the fuel and then the engine "smokes black" and is considered overloaded.

The stoichiometric ratio to achieve complete combustion for gasoline and diesel are very close, around 14.3:1 or so and gasoline must be burned at that mixture for power, a little richer to protect the machinery, a little leaner for economy. Much change either way and the engine won't run well if at all.

Since a diesel doesn't mix fuel and air like a spark ignition engine, it will run very well at 40:1 mixture as at very low loads or at idle. The mixture sweetens up as the amount of fuel injected increases with load until the smoke point is reached and by that time EGTs are sky high since combustion may not be complete by the time the exhaust valve opens.

-- Edited by RickB at 19:02, 2008-10-16

-- Edited by RickB at 19:02, 2008-10-16

Why does a diesel not need a "correct" fuel-air mixture to run properly while a gasoline engine does? I have always assumed that to get the most efficient "bang for the buck" the fuel and air had to be in the correct ratio regardless of engine type to burn with the most efficiency. But NA 4-stroke diesels have no air adjustment at all. Yet they seem to operate efficiently at all throttle settings even though the air supply is not being altered. So why does this continuously changing fuel-air ratio work?

I'm not challenging your earlier post--- I've known for years most dlesels have no air adjustment--- but I've never understood why it works.
"Why does a diesel not need a "correct" fuel-air mixture to run properly while a gasoline engine does?"

You couldn't ask a simple question could you?*The truth is a diesel does require the "correct" mixture. It just achieves that condition differently.

During compression, the cylinder of a*spark ignition engine is filled with all the fuel and air it will use for the power stroke. Complete and efficient*combustion*of this charge requires the fuel to be vaporized and mixed with charge air in a homogenous mixture at the correct ratio of fuel to air.*Too lean or rich a mixture either won't burn or it will burn poorly. A throttle is needed to reduce the*amount of air introduced to the cylinder at low loads to maintain the correct mixture*with a small amount of fuel.

Fuel is introduced to the cylinder of a diesel just before it is required to ignite and injection continues until the time it is no longer needed. The*fuel air ratio*in and around the stream of injected fuel may be infinite*or zero depending on where the droplets*are in the spray pattern.*Combustion occurs when the droplets vaporize enough to create a local mixture that supports combustion.

That is one of the reasons octane and cetane are at opposite ends of the spectrum. We want diesel fuel to ignite easily and burn quickly because we only put in what we want when we want it.**We want gasoline to ignite and burn slowly since the entire cylnder is full of*the stuff*at the start of compression.*

Since combustion in a diesel cylinder is "local" it burns when and where the correct mixture occurs. Combustion is supposed to be complete long before the end of the power stroke and this expansion cools the burned gases. That is why diesel exhaust is cooler than spark ignition exhaust and diesels are more efficient. The heat of combustion*is*converted to*power instead of warming the world.
The stoichiometric ratio is the chemical ratio to get 100% combination of available oxygen from the air with available carbon and hydrogen in the fuel - with energy released from the breaking of the C-H and C=C bonds in the fuel.

In a gasoline engine, you need to maintain a fuel/air ratio somewhere around stoichiometric in order to get it to ignite and burn rapidly with a spark ignition.* If it's way too rich or lean, you won't get it to ignite at all.* If it's a bit rich or lean, you'll get slower flame propagation, so the burn won't complete quickly enough for its energy to be transferred to the piston.
But for best fuel economy (as opposed to maximum power), you want to be a bit on the lean side.* That is, you want to make certain that there's an available oxygen molecule for every C-H and C=C bond in the fuel so that none go out the exhaust pipe intact.* Don't know if you've read any of the stuff about the GAMI fuel injectors for Continental aircraft engines, but the whole thing there is getting balanced mixtures between cylinders so that you can safely run way lean of rich without damage at lower power settings.

Diesel's another story, however.* You're always running way lean of stoichiometric.* When the injector pops, fuel is introduced into air that is hot and compressed enough that it burns almost instantly (with the distinctive diesel "knocking" shound) and completely (because there's plenty of spare oxygen available).* This is part of the reason for the diesel engine's superior fuel efficiency.*

But since you don't need a flame front to spread quickly throughout a fuel/air mixture, you can get by with "excess" oxygen.*

As far as the clogged injector ramifications, Rick is spot on.* A single*partially clogged injector in a gas motor causes that cylinder to run lean, and lean mixtures cause excess heat because of the slow flame propagation.* Lean mixtures are also more prone to detonating.* So that cylinder can run hot enough to do damage.* (An excessively rich mixture also causes slow flame propagation, but the excess fuel acts as a coolant - reference taking off at full rich mixture on piston aircraft engines).

But a single clogged injector on a diesel engine simply causes that cylinder to produce less power.**The scenario that is similar to a clogged injector in a gas motor is an injector providing too much diesel - usually the wrong injector installed, especially in DD engines.* Now one cylinder is overfueled, probably not enough to*make enough black exhaust to notice, but*enough to put a hole*through a piston top eventually.

I'm not real good at explanifying this stuff, I'm afraid... let me know if this makes any sense and I'll try*to do better if it doesn't.*

Whew!* Rick and I sent reponses at almost the same time, and I didn't even contradict anything!!

One nit to pick on octane, however.* Flame propagation speed doesn't change with octane in gasoline engines.* Octane is an indication of the resistance of a fuel/air mixture to detonation, which is spontaneous combustion due to temperature and pressure.* When detonation takes place, instead of a smooth, controlled flame front, you literally get an explosion.*

When things work correctly in a gas motor, the spark initiates combustion.* This starts as a pretty small event right around the spark plug, and is usually initiated 20 degrees or so before the piston hits top dead center.* The flame front is kind of a half sphere radiating from the plug, and its*size increases cubicly with time (just as the volume of a sphere increases as the cube of the radius).* Ideally,*the transition between "some" and "most" of the fuel/air being burned happens shortly after TDC.* This gives peak cylinder pressure starting about 20 degrees ATDC, which is where the mechanics allow a lot of power to be transferred to the crank.

With detonation, the initial burn just after the spark causes cylinder pressure to increase to the point that*part of the fuel/air mixture in some other part of the cylinder begins to combust prematurely.* This increases the pressure still more, so very quickly the entire cylinder charge combusts around or prior to TDC.* Now there's extreme pressure and temperature in the cylinder with nowhere for the energy to go until the crank turns another 20-30 degrees.* The pressure stresses the engine (and makes the knocking sound) and the heat transfers to the piston and the head - where it does no good.

Anyway, most of this is irrelevant to the diesel trawler crowd, but it's cold and raining right now, so I figured I'd bore y'all with it anyway.*
It'a amazing to realize how much we don't know when we realize how much we don't know. Actually it's a bit depressing, like going to Barnes & Noble and seeing all those books about neat stuff I'll never have time to read.

Thank you both Rick and Chris for your explanations. I think I understand the basic difference between gasoline and diesel ignition and how they each make use of the air they are being sent. In short--- and do correct me if I have it wrong--- a gasoline engine needs the correct fuel-air mixture sent to the cylinders where the diesel "creates" the correct fuel-air mixture in the cylinder because of the way the injectors work and the nature of the combustion of diesel fuel.
"This is true of a gas engine as well."

Sorry Vinny, it aint' so. If it were there would be no need for a throttle on a gas engine. While the volume of air in the cylinder may be the same, the weight of air in the cylinder varies widely with throttle setting.*

The throttle restricts the flow of air into the manifold. This restriction lowers the pressure - it creates the famous "vacuum" that is a measure of gas engine performance.* Throttling the air supply*is the only way to maintain a useable fuel air ratio from idle to full power. The fuel supplied drastically changes and the air supply also has to change proportionally.

"I still think that a temp difference between cyls*can indicate a faulty injector"

That is absolutely true. The point being made to Marin is that lack of fuel does not increase the exhaust temperature of a diesel, it will decrease it and "mixture" is not particularly relevant.
"... a gasoline engine needs the correct fuel-air mixture sent to the cylinders where the diesel "creates" the correct fuel-air mixture in the cylinder ..."

That's about it. A gas engine has to get it right with the entire charge from the beginning or it won't work well or at all.

In the combustion chamber of a diesel, mixture happens. That is why there is such intensive research going on in the science of combustion and the art of spray patterns. It is one of the reasons common rail has come back with phenomenally high rail pressures and electronic control of injection timing. With these systems we can modulate the timing and amount of fuel as well as deliver it in multiple injection events during the same power stroke and even intersperse fuel with water to control NOx.

This is the sort of thing that I believe will keep diesels around long after gas engines have gone by the wayside for many applications.
"When the injector pops, fuel is introduced into air that is hot and compressed enough that it burns almost instantly (with the distinctive diesel "knocking" shound)"

So then why do the old mechanical pump engines make the loud distinct knocking sound and the new high pressure common rail systems make such a smaller or almost non-existant knock? And why do different manufacturers diesel engines knocks sound different?

"So then why do the old mechanical pump engines make the loud distinct knocking sound and the new high pressure common rail systems make such a smaller or almost non-existant knock? And why do different manufacturers diesel engines knocks sound different?"

It's not the pump, it's the engine design. Engines that use a direct injection system where the injector squirts directly into the cylinder tend to be be noisier because all the fuel is injected at once*- not really at once but in human scale it is close to all at once*- and tends to combust at all once and make a lot of noise.

Engines with precombustion chambers give the fuel a chance to light off smoothly and make less noise but are less efficient than direct injection engines. Different manufacturers use different piston crown shapes so combustion chamber turbulence is different and this effects the rate of combustion and pressure rise.

Common rail engines can use direct injection for its efficiency but inject a tiny*drop of pilot fuel to heat things up so that the main squirt ignites smoothly and evenly to reduce the rate of pressure rise which keeps th enoise own and also helps to reduce emissions, particularly NOx.
Okay, I think I am following you now......but now explain how a turbocharger affects the combustion scenario of a diesel engine since, ultimately, as technology goes on, almost all newer diesels will be TCed.* ANd I think I have a good handle on gasoline engines and how turbocharging can cause detonation due to high cylinder pressures.* Why is it that diesel engines seem almost "predisposed" to turbocharging(I think I am getting the picture but I am having a problem getting it all together)?

-- Edited by Baker at 15:12, 2008-10-17
"... now explain how a turbocharger affects the combustion scenario of a diesel engine ..."

It effects the combustion only so far as it provides more charge air the cylinder which in turn allows the combustion of more fuel which of course produces more power in the same time period.

Diesels are a natural for turbocharging as you mentioned. They are more heavily built than a gas engine*and the fact that fuel is not introduced until it is time to burn it eliminates many of the temperature induced problems which plague high*output gas engines.

A normally aspirated engine always has less than a full load of air in the cylinder, when running, because of inlet restrictions and friction along the manifolding. So*the engine starts out thinking it is above sea level as far as power output is concerned.

A diesel benefits particularly well from turbocharging because, as I mentioned before, the more air you put in the cylinder the more fuel you can burn and the more power you can extract. The more power extracted the more exhaust gas produced and the more power available to compress the charge air ... power that does not have to come from the output shaft.

Unlike a gas engine of the same power output, the exhaust of a diesel is much cooler so we don't have to deal with red-hot glowing exhaust systems and all the problems associated with high heat rejection. More of the heat from combustion is converted to power in a diesel than a gas engine, that's why they are more efficient.
So much info out of one group. We are going to the boat tonight and I will try Marin's suggestion about one engine running at a time, etc... I have not purchased a tach yet- but will need to do so soon. I will also dig out the Lehman book this weekend to find the idle settings.
Nothing to do with marine diesels but some people like John might find this interesting if they don't already know it. At the end of the heyday of piston engines for transport aircraft the two most powerful engines available were the Pratt & Whitney R-4360 Wasp Major and the Wright R-3350 Duplex Cyclone (the numbers are the total cubic inch displacement of the engines). Both engines developed well in excess of 3,000 hp, with the final version of the Pratt engine cranking out 4,300 hp and the Wright engine coming in with 3,700 hp.

The Pratt engine developed its power with lots of cylinders--- 28--- arranged in four radial rows of seven cylinders each. At first a single mechanical supercharger provided air boost but later two turbochargers were used in addition to the mechanical supercharger.

The Wright engine had two radial rows of nine cylinders for a total of 18. But instead of mechanical superchargers or turbochargers boosting the air feed to the engine, the final versions of this engine used three huge turbochargers that were coupled directly to the crankshaft (well sort of directly, they used fluid drives). So the power generated by the turbochargers was not used to compress air but was used to physically augment the power from the 18 cylinders on the crankshaft.

The Pratt engine was used on a number of airplanes including the B-50 (post-war version of the B-29), the Boeing KC-97 and its civilian counterpart, the Model 377 Stratoliner, the B-36, and a post-war, hot-rod version of the famous Vought Corsair. The Wright Turbo-Compond version of the R-3350 was used on planes like the DC-7, the Lockheed Constellation, and the P-2 Neptune.

Both these engines represented the absolute maximum power that could be wrung out of an air-cooled aircraft piston engine with any degree of reliability (both of them were notoriously trouble-prone) and fortunately for the flying public*they fell out of favor relatively quickly with the introduction of turboprop and turbojet engines.

-- Edited by Marin at 17:19, 2008-10-17
"But instead of mechanical superchargers or turbochargers boosting the air feed to the engine, the final versions of this engine used three huge turbochargers that were coupled directly to the crankshaft (well sort of directly, they used fluid drives)."

They are actually Power Recovery Turbines or PRTs. Those fluid drives worked very well as oil centrifuges too. But they certainly did recover power. The engines do use a mechanical supercharger to deliver charge air. It is a gear driven 2-speed centrifugal blower.

Gawd those were the days of mighty engines and wonderful noises and smells! There's nothing like a radial unless you are westbound over the Rockies in heavy icing then the whine of a turbine doesn't seem quite so effeminate.
RickB wrote:

*The engines do use a mechanical supercharger to deliver charge air. It is a gear driven 2-speed centrifugal blower.

You're right, I forgot that.* The power recovery blowers got all the publicity
* When I moved to Hawaii the Navy had a fleet of radar picket planes on the south ramp of Honolulu Airport.* They were part of the Cold War DEW line--- they would fly out and hold station for hours and hours*over points in the Pacific watching for incoming Soviet bombers.* Whenever they fired these things up (they had the Wright R-3350 engines in them) the entire airport would disappear in a fog of blue-white smoke.* You could see it from downtown Honolulu (no high-rises to block the view*in those days).

The other thing I remember from my countless flights to the mainland and back as a kid*on Stratocruisers and DC-7s was*the ENDLESS engine warm-up before takeoff.* I think they spent as much time warming the engines up as they did making the flight.

And now before John cuts us off we should get back to talking about marine diesels.......


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I checked my Lehman OM and parts manual and you are right. After break in 600-700 RPM is the spec. These old tachs are probably way off- I better get the phototach before I get down there with my toolbox!
I don't know what kind of tachs you have in your boat. In our '73 GB they are Stewart Warner, and while they are reasonably in the ball park you can tap on the glass and change them by 100 rpm or so
I use all the gauges on our boat as trend indicators rather than actual measuring devices. A previous owner marked all the "normal" readings on all the gauges with thin strips of colored tape. Since the boat ran for several decades at these settings before we got it, we figure as long as the needles are always near the tape marks, we're good.
I was Power plant officer on P2's in a patrol squadron, for a couple of years , 3350 time!

IF we got 2000hours between change outs we were really happy .

But much of the damage was due to Navy stoopidity (suprise!).

Each and every TO was done at full power , as you would use getting off a 3500ft Nav Fac (a small island) even tho some runways were 8000, or 9000 ft long.2900RPM , 52.5 in of manifild pressure with alchohol and water injection, 5 min max.

Only in the air line did I learn the secret of TO by Da Book with much reduced power settings.

But they sure did sound good at TO.

The best was on Command changes , we would TO as a section , 4 planes going at once.

Of course it was illegal by Navy rules

, BUT the old CO wasn't going to NOT hand over the squadron , just to raz some junior officers,and the new CO had the problem of it" didn't happen on his watch".

Fun for the feeble minded maybe , but great fun indeed.

-- Edited by FF at 06:03, 2008-10-18
Something to keep in mind about idle speed. Over proped boats have considerally more thrust at idle speed and consequently require shifting in and out of neutral gear at dead slow speeds much more than correctly proped boats. To make an over proped boat eaiser to live with at very slow speeds one should get the idle speed as low as possible. Thanks Marin, I remember those 3350s both While flying to Alaska in the late 50s on the Connies and in the Navy ( P2s & P5s at VP31 in San Diego ). Saw a Connie at a fly-in several years ago and it seemed tiny. I remembered them as huge .. but it didn't seem very large then. It was a restored MATS bird. As for Deisel and gas engine efficency I heard or read they are almost the same at WOT. By the way this is the first post using my new computer .

Eric Henning

-- Edited by nomadwilly at 14:13, 2008-10-18

-- Edited by nomadwilly at 14:14, 2008-10-18
Hmm, that "same efficiency at WOT" sounded pretty suspicious, so I did a little looking around (ain't the internet a wunnerful thing).

Turns out that the gas motor manufacturers (Crusader, Mercruiser, Volvo) don't seem inclined to publish fuel consumption curves like the diesel mfrs do.

But I found a boat test on the Mercruiser site that gave fuel flows at WOT.

So:* a Mercruiser 8.1HO is a 425 HP 8.1 liter gasoline engine (I don't know who makes the 8.1 block - GM I think - but it's the same big block for all three of the gas motor manufacturers).

The Cummins Mercruiser QSB5.9 is a "modern" high output 425 HP diesel motor of 5.9 liters (interesting that it's smaller displacement than the gas motor).

At WOT, the diesel motor is pulling in 22.5 gal/hr.* This is in line with the rough number of 1 gal/hr per 20 hp on a diesel motor that I've always heard as a rule of thumb.

Now hold on to your wallets.* The 8.1HO gas motor pulls 33*gallons per hour at WOT.* This is a bit better than the old rule-of-thumb that I've used of 1 gal/hr per 10 hp on a gas motor - probably because the new motors have EFI, electronic spark control, and such.* This is running closer to 1 gal/hr per 13 hp.

It is true that the diesels are better at low output - especially idling.* Gas motors have to run rich at idle to allow the spark to initiate combustion, while the diesel motor quite happily burns everything at idle because it's waaay oxygen rich.*

So I dunno where you heard or read that they were similar efficienty at WOT - tho it might be true that they have the least difference at WOT.
Some of the diesel /gas difference is simply the weight of the fuel choice.

The rest is due to the compression ratio, the diesel piston gets to recieve the power harder and longer on the power stroke.

Absolutely.* Diesel is about 7lb/gal vs 6 lb/gal for gasoline.* Proportionally more carbon and less hydrogen in each molecule - and ripping apart a carbon-carbon bond produces more energy than a hydrogen-carbon bond.
While there is a difference in the weight per gallon of diesel and gasoline, the heating value per pound is fairly close for both fuels at around 18000 to 19000 btu/lb. The percentage of carbon by weight in diesel and gasoline is within 1 or 2 percent.

To make a useable comparison you have to calculate the amount of power produced per unit of weight of fuel burned. That figure is called Brake Specific Fuel Consumption (BSFC) and for the 425 horsepower engines cited, the diesel is the clear winner with a BSFC of .38 pounds per horsepower per hour (lb/hp-hr) while the gasoline engine burned .48 lb/hp-hr.

Those figures are based on average weights per gallon for diesel at 7.3 lb/gal and gasoline at 6.2 lb/gal.

-- Edited by RickB at 20:43, 2008-10-20
Probably should have kept my mouth shut .. been there before. I looked but I couldn't find it. Found a book ( Power for the Small Boat - Tod Mead - 1947 ) where I thought I'd seen it but could'nt find. Interesting .. the cost of gasoline then was .20 gal and diesel was .08. put another spring line on my boat tonight as winds are expected to reach 60 knots tomorrow.

Eric Henning
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