Lehman Idle Speed

[Marin]

Rick and Chris---

Another question..... I understand (basically) the bit about a gasoline engine needing the correct fuel-air mixture fed to it while the diesel "creates" the correct fuel-air mixture in the cylinder because of the way fuel injectors work and the way diesel fuel burns.

So.... what about fuel-injected gasoline engines? Do they operate in a similar way to a diesel? In other words, in my car am I (or the computer) regulating the fuel flow to the injectors while the cylinders simply get all the air they need all the time to support whatever "burn power" occurs as a result of the metering of the fuel to the injectors?

 

[RickB]

"So.... what about fuel-injected gasoline engines?"

All the fuel burned in a diesel is injected at the end of the compression stroke, the intake valves are closed, and the air is under high pressure.
*
In a fuel injected gasoline engine, the fuel is mixed with the incoming air during the intake stroke. The fuel may be injected at the throttle body or the intake ports. In*either case*air entering the cylinder contains the fuel mixture before the start of compression.

-- Edited by RickB at 06:25, 2008-10-21

-- Edited by RickB at 06:26, 2008-10-21

 

[Jim Spence]

In most fuel injected gas engines, the mixture ratio (fuel to air) is controlled by the computer by increasing or decreasing how many miliseconds the fuel injector is turned on (pulse width). It determines injector time by input from the Mass Air Flow sensor, Throttle position Sensor, and Oxygen sensor. (older units use a vacuum reading instead of a MAF sensor). Spark timing is also controlled by the computer using input from Knock sensors, crankshaft sensors and cam sensors.

 

[Nomad Willy]

Rick, Sloboat,
Do*some FI 4 stroke engines have injectors that inject the fuel directly into the cylinders? Marin, are there aircraft engines that are directly injected? Is my Evinrude " E- Tech " OB directly injected ? They call the Tohatsu " DFI " direct injected. Or do they all just squirt the fuel into the intake ports?

Eric Henning

-- Edited by nomadwilly at 17:00, 2008-10-21

 

[Nomad Willy]

Hey Eric,
Funny you should mention the 300 SL .. My friend Larry had one of those Gull Wings and at the same time I had a 140 MC Jaguar roadster. I had complete strangers ( girls ) jump in the car at a light. Must have been the car that caused the strange behavior*as it never happened with any other car. I drove the Gull Wing and to my supprise it was not fun .. unless you were driving really hard. Even at slow speeds it took both hands on the wheel to negociate a corner .. then you could shift. Both cars could make 150 mph .. fast for the late 60s. Cheers and thanks.

Eric

 

[Chris Foster]

So should this thread get the award for "simplest question that turned into the most technically involved discussion" on TF???

 

[Jim Spence]

I don't own a Lehman, I kept watching this thread grow and finally when it hit 4 pages, I couldn't imagine what you guys could be talking about, said to myself, "How in the hell could it possibly take 4 pages to explain how to set the speed on what I have always thought of as a small diesel?"
And I'm damned if you guys didn't suck me in too!!
Have a great day!!

PS We are leaving this morning to start about a 6 month cruise, will keep in touch!!

 

[Forkliftt]

OK- so I think I understand now.... If I want to speed the idle up I go righty tighty on the screw!!

 

[Marin]

Forklift---

If you are trying to figure out how to adjust the idle speed on a Lehman 120 and don't have an operators manual (which explains how to do this) you can download the FL120 operators manual from the Grand Banks Owners website. http://www.grandbanksowners.com/index.php *There is a "Manuals" section in the overall menu on the left side of the homepage. I believe that in order to access the Manuals section you have to join the owners forum, but there is no charge for this. The Manuals section also contains manuals for other engines and transmissions like the ubiquitous BW Velvet Drive.

-- Edited by Marin at 12:29, 2008-10-23

 

[Penta]

Great information but now I have a curve for you. I have an old Volvo diesel in our boat (MD-47 series) and the manual shows the engine having both a "Throttle Body" on the intake manifold, containing the usual butterfly valve, as well as a "Control Arm" on the injector pump. My particular engine does not have the "Throttle Body" on the intake manifold and I can not understand what it would do if it was there as in my way of thinking there would have to be a very complex array of levers etc in order for the air throttle and the injector throttle to operate together efficiently. Unfortunately there are no diagrams showing how this all hooked up and my engine works just fine thank you using only the injector pump control like every other diesel I have ever seen.
Anyone care to hazard a guess as to what the "Throttle Body"would do if it existed? It does not appear to be a "Shut-Down" device as it has several holes in the butterfly valve.

Confused

John Tones MV Penta
Sidney, BC

 

[Chris Foster]

Hmmm..

If there is a throttle plate, the mechanical interconnection between that and the fuel control isn't an unreasonably complex mechanism.* Light aircraft fuel injection systems use that very system.* Of course, most light aircraft engine systems are 40 to 50 year old designs, but don't get me started...

There are a couple of possibilities that I can think of.* The air cutoff would be one possibility.* The other (this is purely a WAG) is that they might want to decrease the air charge a bit at near-idle settings to improve idle smoothness.* At idle, you don't need much air to still have a very oxygen-rich burn.* But having enough momentum to get that piston pushed up against a 17-or-so-to-1 compression ratio can be a problem at low idle speeds.* Either you need a massive flywheel, or I suppose decreasing the air charge might help (so long as it stays sufficient to generate enough heat to light off the fuel when it's injected).

MD-47 is really the model number?* I can't find any references to that on boatdiesel.com.* What are its features (# of cylinders, HP, turbocharged, ??)?* About when was it manufactured?** (Inquiring minds want to know!!)

 

[Penta]

Cris - yes that is the correct model number and no you will find very little in the way of references to it. It was part of a series of marine engines that Volvo produced through the 50's and 60's and a whole lot of them found their way into commercial fishing boats up here.
There were 4 engines in the series, the MD-47 being the smallest followed by the MD-67, MD-96 and the TMD-96. They are all natural asp. except for the TMD-96 which was turbo boosted.
The MD-47 produces 91 HP / 2500 RPM and is a 287 cu/in displacement and the T96 was 175 HP.
Parts are very hard to impossible to locate in North America however I have been lucky to locate some parts such as gaskets and valves in Switzerland as the 47 is very similar to a fixed power plant engine they also made and which is still used in large machinery and generator plants.
In our 36 foot ex fish boat I get a cruising speed of 6.8 knots with a burn rate of 1.25 GPH so she is well worth trying to keep.
I did a "head job" on it about 500 Hrs ago and at that time it was sitting at just over 8500 Hrs since it was totally re-built in 1980. The liners were in great shape as were the valves so all I did was de-carboned the heads and put her back together again. A nice feature is that there are 2 access plates on the port side of the engine which allow you to access that crankshaft and actually service the bearings if required. An oil test revealed very little in the way of metal particles so I have left that area alone with the exception of "swabbing" out the oil pan. She starts within half a dozen revolutions cold or hot and smokes very little when cold so how can I convince the Admiral that we need a new engine
Hope this answers the inquiring mind but if not I have the manual and can quote lots of data.
John Tones

 

[Chris Foster]

Hmmm... no flashes of insight, except that I'd guess we're talking about a 4 cylinder engine, so perhaps the idle smoothing in some applications.

I'll put out*a query on boatdiesel and see if I get any bites.

 

[Penta]

Chris - sorry, I forgot that one fact, its a 6 cylinder engine as is the whole series. Like I say my engine only has the injector control for engine speed but seeing the air control in the manual is what has me intrigued.
I was just looking very closely at the pictures in the manual (even with a magnifying glass) and it seems as though Volvo used ONLY the air control for throttle control on the earliest engines in the series. I also have a manual for a later version of the MD-47 and it shows the injector control as I have but the early manual shows only the air and there is no sign of any control on the injector pump ???? To make matters more confusing the early manual, in the section on the injector pump, shows the throttle control, but not in the opening photos of the series. Maybe the very early engines used the air control but later versions did not and only the injector section of the manual was changed. Again, who knows ??
John

 

[RickB]

"Great information but now I have a curve for you."

Is the injection* pump a Bosch M60 by any chance? In the early '60s several diesel manufacturers used a pneumatic governor control that took its signal from the inlet manifold. The throttle plate was the speed control, when the plate was partially closed the vacuum created would reduce the amount of fuel injected, when the plate was open the vacuum was reduced and more fuel was injected.

These things would only work on an engine that worked with a linear relationship to load and speed such as a fixed pitch propeller or a forklift truck.*

 

[Penta]

Rick - by gosh I think you have solved it. The "Throttle Body" has a pipe fitting coming out of it on the engine side of the butterfly valve and a tube running somewhere around the rear of the engine however the pictures do not show things in enough detail to see if it connects to the injector pump. The pump is listed as being a CAV and like I said the pictures show no sign of a "control" arm such as the Bosh pump that I have.
Your explanation makes a lot of sense and fills in the gaps left open in the manual so I think we now have answered my question and I will go away happy !!
Great bunch on this list - thanks much

John

 

[Chris Foster]

Tip of the hat to you, Rick.

Great call.