Mechanical Questions 101

LifeAsBen

LifeAsBen

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LifeAsBen
Hi, new to the forum. Already have found many great posts and have learned a lot. So first I would like to just say thanks to the community for contributing and generating an amazingly useful place!

I'm a low time pilot, 100 hrs. I first got my license over 15 years ago and have only flown seldomly since. I am now actively flying and working towards instrument rating.

Anyway, I've always found it odd that as students we're never really allowed to see and touch anything under the cowl (beyond pulling a dip stick). I feel like a certain amount of hands on mechanical instruction would be beneficial to a private ticket. I realize that everyone has different skills and interests, mine lean towards mechanical so I thought I'd start a thread with my questions (hopefully they'll be useful for others too):

1 - Why are engine rpm's limited so low? Your average car can spin up to 6,000 without issue. As a vintage BMW motorcycle owner I appreciate that we operate our boxer engines at low rpms but even my old beemer will see 5k rpm with no issue. Just curious.

2 - Why do spark plugs seem to foul so often? Our daily cars and trucks never seem to have this issue, or maybe it's just that we never notice it.

3 - Why are we still in the dark ages choosing our own mixture? We trust computers to do almost everything in our modern lives, many of us driving cars with drive by wire braking and power steering. But yet we as pilots (or maybe policy) think we can choose the best fuel / air mixture. I know FADEC is a thing but for the most part (it seems) that all GA training craft are manual. I can only imagine how poor we truly are at choosing the best mixture setting by sound vs. actual data as a computer would.

4 - I rent / fly a fuel injected 172 .The starting procedure is to crank it with the mixture completely leaned and then full rich when it starts to pop. Pulling the mixture fully out also is the procedure to kill it. It would seem that fully leaning it is equal to completely shutting off fuel. So my question is basic, how does it start with fuel essentially shut off? Maybe it's that the throttle is cracked 1/4 inch? But when running and with partially cracked throttle pulling mixture still kills it. On a cold start the procedure requires a 3 second fuel pump burst, and this makes sense. But on a warm start fuel pump is not used so with mixture pulled to full lean you'd think there would be no fuel to burn.

5 - Why are carbs underneath the engine? Simply a matter of fitment, more space underneath or does it serve a purpose? I've overhauled, replaced, all sorts of car and motorcycle carbs - how does an aviation carb differ? Anyone have a diagram?

6 - A tag on to #5, how does an aerobatic carb differ? Or does it? I can't understand how a float bowl (if they have them) would work upside down.

7 - It seems to me most small airplanes have 24v systems. Why?

8 - Why a split master switch? I assume it's so one can be eliminated in the event of a failure?

9 - Why doesn't leaning cause engine components to fail due to the excessive heat? If you leaned a car engine past peak my car buddy ventured that it would blow a hole in a piston.

More questions to come, or add your own...
 
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1. Our engines are direct drive and the prop tip speeds limit our ability to run high RPMs when a prop goes supersonic it looses its efficiency.

2.They should not, you need to lean.

3 Our engines require a major change when converting to modern fuel feed methods. that require lots of testing and engineering approvals, the market is too small to off set the costs. plus the cost of parts to update will be higher than any owner would pay.

4. try starting this way. set throttle at 1/8th inch, pull primer out and allow to fill, turn mags on (both) and as you start cranking push the primer in. it should start right up.
 
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1. Props are typically direct drive. Spinning the engine faster causes the prop to turn faster. When the tip speed goes supersonic the prop becomes less efficient. Using a gearbox to keep the prop speed down has proven unreliable.

2. Leaded gas, oil from looser tolerances from being air cooled, and and a weaker spark.

3. Most aircraft are from before FADEC/computers and old mechanical solutions were unreliable. New aircraft are not being built in significant numbers to justify certification costs.
 
tyndall said:
Using a gearbox to keep the prop speed down has proven unreliable.
Click to expand...

Not so, almost all large radial engines are geared, as was the GO-720, GO-480, GO-300 and several others, and when operated properly they run as well as any other.
the GO-300 gave the geared engine a bad name simply because the pilots of the Cessna 175 would not run the proper RPS which would lug the engine and cause cylinder failures.
 
tyndall said:
New aircraft are not being built in significant numbers to justify certification costs.
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What would you call ROTEX engines?
 
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I fly behind a fadec engine,the computer does all the work.
 
If the engine is flooded, you dont want to add fuel. Thus the full lean without priming. You are igniting gas that is already in the system. If it isn't flooded it wont start with a flood start procedure, you will have to prime it.
 
Skipping 1 and 3 because they've been answered pretty good.
4 - our arrow seems to work best like this: cold start- throttle 1/2", fuel pump ON, (using the mixture since we have no 'prime'knob) prime, mixture to full lean, crank and advance after start. After fueling/hot start - throttle open 1", release the pressure in the fuel lines using the mixture (full rich, watch the fuel flow needle move, full lean), throttle to 1/2", crank.
The hot start uses what fuel is left in the lines to prime the engine. Seems to work well for us but YMMV.

In regards to your second question about aviation plugs fouling. Is it me or would the shape of the plug (if spark plugs were belly buttons cars look like outies, aviation look like innies) allow carbon to build up easier?
Obviously leaning helps tremendously.
 
Tom-D said:
Not so, almost all large radial engines are geared, as was the GO-720, GO-480, GO-300 and several others, and when operated properly they run as well as any other.
the GO-300 gave the geared engine a bad name simply because the pilots of the Cessna 175 would not run the proper RPS which would lug the engine and cause cylinder failures.
Click to expand...

GO-720? Surely you jest. The largest geared engine Lycoming made is a GO-580. The problems is one of diminishing returns. The efficiency gain in the reducing the propeller speed is frequently lost in the additional weight of the gearing. Add in the complexity of the gearing, the reliability problems with gear wear (admittedly redcued by proper pilot procedure) and the problems with certain accessory systems (like constant speed prop systems) in the geared environment make it a hard sell.


As pointed out the major reason you see fouled plugs are bad pilot technique. The reason you don't see the average schlub fouling his plugs in cars is because mags are a weak spark (as others have pointed out). Even the older car coil/ignition/points system is stronger, the modern electronic ignition can burn through absolute crud (I had a distributor cap that was pretty much destroyed but the plugs still fired). The bigger question is why there aren't better aviation ignition systems in common use.
 
Thanks for all the replies.

My #4 question is confusing. It's probably a "well duh" question but it still intrigues me. Let me re-ask it: If you haven't primed and mixture is at full lean AND presumably all fuel was burned up at engine shut down (b/c mixture was pulled out to kill engine) how is the engine starting? Maybe the answer is simple: my assumption that full lean cuts ALL fuel is incorrect. So with throttle cracked and mixture leaned the engine gets just enough to begin to pop.
 
flyingron said:
As pointed out the major reason you see fouled plugs are bad pilot technique. The reason you don't see the average schlub fouling his plugs in cars is because mags are a weak spark (as others have pointed out). Even the older car coil/ignition/points system is stronger, the modern electronic ignition can burn through absolute crud (I had a distributor cap that was pretty much destroyed but the plugs still fired). The bigger question is why there aren't better aviation ignition systems in common use.
Click to expand...

<< hmmmm, great info. The more I learn the more I'm disappointed about how far our engine technology is behind modernity. Yet we have the most sophisticated cutting edge navigation and cool electronic gadgets in our panels.
 
The full rear travel of the mixture control is called "IDLE CUTOFF". At very low power settings (idle), the normal fuel-air mixture setting (the red knob) is not used. A not-user settable mixture is used. The ICO indeed shuts off all the fuel there. However, just because the engine dies because there isn't enough fuel for continued running doesn't mean that all the fuel in the lines is consumed. That's why hot starts are hard. There is enough fuel left in the lines to vaporize and prevent fuel flow.
The second reason is that I can almost guarantee your hot start procedure involves PRIMING the engine. This introduces fuel into the system so even though you're cranking with the fuel cut off, there's enough there to start the engine.

Yes, you have to advance the mixture quickly once it catches (and retard the throttle) to keep it going.
 
Added a few more questions to original post:

5 - Why are carbs underneath the engine? Simply a matter of fitment, more space underneath or does it serve a purpose? I've overhauled, replaced, all sorts of car and motorcycle carbs - how does an aviation carb differ? Anyone have a diagram?

6 - A tag on to #5, how does an aerobatic carb differ? Or does it? I can't understand how a float bowl (if they have them) would work upside down.

7 - It seems to me most small airplanes have 24v systems. Why?
 
If you pressurize the fuel system and move the mixture away from idle cut-off the servo will deliver fuel even though the engine is not running. Flooded city. http://www.n2999c.com/N2999C-info/aircraft/Skyranch_Information/rsafuelservo.pdf

Not all aircraft have updraft carburetors. But they are convenient in that they drip fuel on the ground and not on the engine.

Carburetors for inverted fuel do not use a float bowl - instead there is a regulator diaphragm to control the fuel pressure supplied to the metering jets.

Some aircraft have 24 volt systems because that lets you use skinnier wires (twice the volts = half the current for the same power) and save weight. But I don't know that "most" have 24 volts.
 
7 - Higher voltage = lower current draw, so starter wiring (and the starter motor itself) can be smaller and lighter than the 12 V equivalent. A better question is why aren't all aircraft and cars 24 V systems.
 
Also, regarding Q4, the starting procedure for a FI engine can vary with the engine. Most Lycoming FI engines are started at idle cutoff. Most Continental FI engines are started with the mixture full rich.
 
Carbs are on the bottom in case they catch fire. Fire doesnt get to the cockpit as easy from there.
 
LifeAsBen said:
<< hmmmm, great info. The more I learn the more I'm disappointed about how far our engine technology is behind modernity. Yet we have the most sophisticated cutting edge navigation and cool electronic gadgets in our panels.
Click to expand...

Believe it or not, on a BSFC (brake specific fuel consumption, a measure of fuel flow per power output) basis, aircraft engines are fairly efficient despite the old technology and low compression ratio. Someone compared the BSFC for a 1.9L Saturn engine (0.68 lb/hp/hr) to a Lycoming 0-320 (0.77 lb/hp/hr). The car engine is more efficient but it's not a big difference, and if you consider that the Saturn engine doesn't spend a lot of time at it's best efficiency point, which is wide open throttle at highway speeds, and the aircraft engine operates at or close to it's best efficiency point under normal conditions, the aircraft engine looks pretty good from a fuel efficiency standpoint.

http://michaelsoroka.com/2014/03/26/are-airplane-engines-inefficient/
 
flyingron said:
The full rear travel of the mixture control is called "IDLE CUTOFF". At very low power settings (idle), the normal fuel-air mixture setting (the red knob) is not used. A not-user settable mixture is used. The ICO indeed shuts off all the fuel there. However, just because the engine dies because there isn't enough fuel for continued running doesn't mean that all the fuel in the lines is consumed. That's why hot starts are hard. There is enough fuel left in the lines to vaporize and prevent fuel flow.
The second reason is that I can almost guarantee your hot start procedure involves PRIMING the engine. This introduces fuel into the system so even though you're cranking with the fuel cut off, there's enough there to start the engine.

Yes, you have to advance the mixture quickly once it catches (and retard the throttle) to keep it going.
Click to expand...

Another way to to look at the starting procedure, it turns out that the fuel/air mixture has to be pretty close to correct to get the ignition of the mixture. To rich or to lean it won't ignite. Close, it may ignite but will will either explode(lean=detonate) or burn very slowly (rich) causing very little power during the power stroke. Rich also has the advantage of retarding the ignition, which is useful for starting. The solution is to start with a very rich mixture(Prime) as you start cranking and let it lean out until the correct mixture is obtained. So Prime, but not more fuel (idle cut off) and start cranking, when it starts firing start adding fuel (increase mixture) to maintain fuel/air mixture.

I also suspect that idle cut off often doesn't fully shut the fuel off, it only reduces it to the point the engine will not run. So with a Hot engine fuel will vaporize(expand) and pressurize the fuel lines pushing fuel into the intake. Thus the reason some engines do not requiring priming for starting when Hot.

Brian
 
For an injected Lycosaurus 320-D1A, my hot start procedure that works **most** of the time is thus:

No prime at all with electric pump. Mixture to ICO, throttle cracked open slightly. Start cranking and slowly start pushing the mixture in. It usually fires after about 1/3 of the mixture knob's stroke. This works well when the engine has been stopped for 10-15 minutes, such as a fuel stop.

If this fails, I push both knobs full in, hit the aux. fuel pump for about 1.5 sec., mixture to ICO, throttle cracked open, then proceed like a cold start.

On the spark plug fouling question, these 1930s-tech air/oil-cooled engines have pretty sloppy tolerances compared to a modern automotive engine, so you're going to get more blow-by and oil fouling of the plugs, especially with an engine nearing overhaul. And the blow-by works both ways...the oil gets contaminated by combustion gases sooner, hence the fairly frequent oil change intervals. Also, aircraft engines are just worked harder, considering that in cruise you're at perhaps 60 to 65 percent power, where a car needs maybe 15 to 20 percent power to maintain 70 mph on the Interstate.

Also, they're grossly oversquare (bore much larger than stroke), so big heavy pistons are going to put a damper on high revs. Fortunately, 2700 rpm or so does the trick.
 
5. Visibility. If the carb was on top you'd have to sit higher to see over it, which increases frontal area and drag. Look at radial powered aircraft and how having so much above the crank centerline affects visibility.

6. A float carb can work for aerobatics as long as you keep positive g's. There's also fuel injection.
 
Tom-D said:
4. try starting this way. set throttle at 1/8th inch, pull primer out and allow to fill, turn mags on (both) and as you start cranking push the primer in. it should start right up.
Click to expand...

OP stated he was using a fuel injection motor, no primer to fill.
 
SoCal RV Flyer said:
For an injected Lycosaurus 320-D1A, my hot start procedure that works **most** of the time is thus:

No prime at all with electric pump. Mixture to ICO, throttle cracked open slightly. Start cranking and slowly start pushing the mixture in. It usually fires after about 1/3 of the mixture knob's stroke. This works well when the engine has been stopped for 10-15 minutes, such as a fuel stop.

If this fails, I push both knobs full in, hit the aux. fuel pump for about 1.5 sec., mixture to ICO, throttle cracked open, then proceed like a cold start.
.
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Check you fuel pressure gauge, if you still have pressure, follow first paragraph, if not it's vaporized out and do the 3rd paragraph.

My understanding is Lycomings don't circulate the excess fuel, TCM have a return line, hence the difference in techniques. Also ICO doesn't completely cut off the fuel, so when enough fuel gets through, it starts.
On hot starts if it sounds rough, hit the fuel boost pump and rev the engine a little to clear it of half vaporized fuel.
 
teejayevans said:
On hot starts if it sounds rough, hit the fuel boost pump and rev the engine a little to clear it of half vaporized fuel.
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I'll have to try that. It does run quite rough on the hot start for about a minute. I'll let you know how it goes!
 
Sac Arrow said:
Believe it or not, on a BSFC (brake specific fuel consumption, a measure of fuel flow per power output) basis, aircraft engines are fairly efficient despite the old technology and low compression ratio. Someone compared the BSFC for a 1.9L Saturn engine (0.68 lb/hp/hr) to a Lycoming 0-320 (0.77 lb/hp/hr). The car engine is more efficient but it's not a big difference, and if you consider that the Saturn engine doesn't spend a lot of time at it's best efficiency point, which is wide open throttle at highway speeds, and the aircraft engine operates at or close to it's best efficiency point under normal conditions, the aircraft engine looks pretty good from a fuel efficiency standpoint.

http://michaelsoroka.com/2014/03/26/are-airplane-engines-inefficient/
Click to expand...

You may want to check those numbers. I've seen numerous references to Lycomings cruising at ~0.45 lb/hp/hr and Continentals doing marginally better.

Here's an AvWeb article that includes some data: http://www.avweb.com/blogs/insider/AreDieselsReallyMoreEconomical_197391-1.html
 
The carb is on the bottom (updraft) so it can produce as much ice as possible. Most Cessna's end their lifes as motel ice makers, especially 182s with carbs.
 
Follow the flow of the cooling air. On many of the Lyc and Conti models the air comes in thru the nose vents and is then directed down thru the cylinder cooling fins. This means the air is warmer below the engine than above it.

Want to reduce the chance of carb ice? Put the carb below the engine and let it breathe warm air.
 
Sundancer said:
The carb is on the bottom (updraft) so it can produce as much ice as possible. Most Cessna's end their lifes as motel ice makers, especially 182s with carbs.
Click to expand...

Cars, when they had carbs, also made carb ice. It was a constant headache until the automakers came out with automatic carb heat. It had a vacuum-driven diaphragm that pulled a flapper valve to shut off the cold air and allow heated air from around the exhaust manifold to be drawn in. Sound familiar? That vacuum diaphragm was controlled by a thermostatic valve mounted inside the air cleaner, which constantly monitored the temperature. All it did was allow a leak so that the diaphragm didn't move the valve unless the temp was below about 70°F. Then it would shut off the leak so manifold vacuum would pull the valve toward the hot position, then modulate it to keep the temp constant. Above 70°F ambient, the carb, which was heated from below by the exhaust manifold or via an exhaust passage in the intake manifold, wasn't likely to ice up. That thermostatic valve kept the air passing through the carb at 70°F or better. Besides preventing ice, it aided in fuel vaporization and better mileage. But not better power, which is one reason why that system isn't used on airplanes.

Anytime you have a pressure drop (caused by the carb's venturi) you will have a temperature drop. Anytime you have an evaporating fluid, especially one with a high volatility like gasoline, you will have a temperature drop as the liquid absorbs heat from the air to change itself to a vapor. Anytime you have a big enough temperature drop when water vapor gets involved, you will get water or ice. Car or airplane; doesn't matter. In summertime, too, which is what gets too many pilots who don't understand carb ice.

Aircraft plugs foul because there's too much fuel when a pilot doesn't lean the mixture, and because there is so much tetraethyl lead in 100LL. Four times as much as the old leaded car gas had.

Fuel injected engines have a fuel distributor (flow divider) on top of the engine where the metered fuel enters and is divided to flow into injector lines to each cylinder. There's a pressure-sensitive valve in there that closes when the mixture control is pulled to idle cutoff, to stop the fuel flow in the injector lines and keep the engine from sucking fuel and running on for a bit. The fuel sits in those lines and is boiled by the heat from the cylinders and dribbles into the intakes, flooding the engine and making hot starts interesting.

Figure2-36.jpg

Serious aerobatic airplanes with carbs use pressure carburetors. No float bowl. Diaphragms sense air velocity and meter the fuel accordingly.

Schematic+of+the+PS+series+carburetor.jpg
 
So Diesel engines have a fuel pump that delivers fuel to the individual injectors, and then there is electrically activated injectors on cars, but for aircraft engines the fuel is just suck in like juice threw a straw?
With more precise fuel delivery and precise (electronic) ignition and higher compressions the 100LL can handle, what is the theoretical efficiency they can accomplish?
 
teejayevans said:
So Diesel engines have a fuel pump that delivers fuel to the individual injectors, and then there is electrically activated injectors on cars, but for aircraft engines the fuel is just suck in like juice threw a straw?
With more precise fuel delivery and precise (electronic) ignition and higher compressions the 100LL can handle, what is the theoretical efficiency they can accomplish?
Click to expand...

It's really not a very big gain. The trick is that aircraft engines are normally operated in a very narrow range of RPM while automobile engines are operated in a wide range. The simple mechanical systems are pretty easy to optimize for a narrow range of operation but not for a wide range of operation. That's why the auto engines have made such dramatic increases-their mechanical systems were always a major compromise for the broad range of power/rpm settings they operate in. If you look above in this thread you'll see some specific consumption figures/hp. They're really pretty good for the engines we have.

John
 
teejayevans said:
With more precise fuel delivery and precise (electronic) ignition and higher compressions the 100LL can handle, what is the theoretical efficiency they can accomplish?
Click to expand...
Compared to what? Compared to the best you can do now, a few percent. Compared to running rich of peak, a few more percent.
To see real improvements, you would also need to change the basic combustion chamber more than you really need to change the fuel delivery.
At the moment, aircraft engines are not limited by emission regulations so with some changes such as introducing swirl / tumble to improve the lean tolerance, you could pick up a couple percent. But, the other issue is that aircraft engines run at high loads, and at higher altitudes they are already at wide open throttle so a lot of the games that the auto industry plays with valve timing and such won't buy you much at all since they are primarily directed at reducing pumping losses during low load operation.
If you want to boost compression significantly, you need to go to smaller combustion chambers and more cylinders. While that would help with knock tolerance, it increases the surface to volume ratio in the chamber which increases heat loss.
Changing spark timing with the fuel ratio would be good if you had a lean tolerant engine that could run out past, say, 20:1. But, again, since the aircraft engine operates in such a limited range for so much of the time, the grand total benefit is less than stellar.
That said, there is room for improvement - just don't expect a miracle.
 
teejayevans said:
OP stated he was using a fuel injection motor, no primer to fill.
Click to expand...
Capt. Geoffrey Thorpe said:
Compared to what? Compared to the best you can do now, a few percent. Compared to running rich of peak, a few more percent.
To see real improvements, you would also need to change the basic combustion chamber more than you really need to change the fuel delivery.
At the moment, aircraft engines are not limited by emission regulations so with some changes such as introducing swirl / tumble to improve the lean tolerance, you could pick up a couple percent. But, the other issue is that aircraft engines run at high loads, and at higher altitudes they are already at wide open throttle so a lot of the games that the auto industry plays with valve timing and such won't buy you much at all since they are primarily directed at reducing pumping losses during low load operation.
If you want to boost compression significantly, you need to go to smaller combustion chambers and more cylinders. While that would help with knock tolerance, it increases the surface to volume ratio in the chamber which increases heat loss.
Changing spark timing with the fuel ratio would be good if you had a lean tolerant engine that could run out past, say, 20:1. But, again, since the aircraft engine operates in such a limited range for so much of the time, the grand total benefit is less than stellar.
That said, there is room for improvement - just don't expect a miracle.
Click to expand...
Lightspeed engineering has done the most that can be done in this area, simply by piston design and compression ratio.
plus the Addition of SDS fuel injection, dual electronic ignition the 0-200 will produce 130-140 horse power.
Lots of engines racing that have this upgrade, yet no response from the FAA you to run this stuff in your C-150.
 
Question 8 added:

Why a split master switch? I assume it's so one can be eliminated in the event of a failure?
 
LifeAsBen said:
Question 8 added:

Why a split master switch? I assume it's so one can be eliminated in the event of a failure?
Click to expand...
One side controls the master contactor. The other controls the alternator by switching the regulator on and off. It's there to shut off the alternator in case the regulator fails in a way that the alternator runs away and overcharges the battery and overvolts everything (at a level less than the alternator breaker's capacity) or if the alternator fails and its field is drawing current from the airplane's battery when more critical systems need it.
 
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