Why the rev drop?

We all know that one of the pre-flight checks we always do is the magneto operation check. "Both - check revs; Left - check revs; Both; Right - check revs." We know that if the difference between the revs at "Both" and those at "Left" or "Right" is about 50 RPM, then the magnetos are working. But why do the revs drop when we go to only one magneto?

 

It's all to do with the combustion of the fuel/air mixture. For too long, people have been taught that the events in the Otto 4-stroke cycle are: Suck; Squeeze; Bang; Blow. But that is wrong. When the fuel/air mixture goes

we have detonation, and that is bad for the engine. It's about time people were taught: Suck; Squeeze; BURN; Blow.

 

We have all seen movies where a flame front travels down a stream of petrol, or along a gun powder trail. More recently we have seen fire fronts moving into unburnt bush. That is exactly how the fuel/air mixture burns in a combustion chamber.

 

The flame is initiated at the spark plug electrode and moves across the combustion chamber, igniting the fresh fuel/air mixture as it reaches it. Here are pictures of the combustion of fuel/air in a combustion chamber. The engine happens to be a side valve engine with the valves located to the right of the right of the picture and the piston to the left.

 

1. The spark is initiated at the spark plug electrode.

 

2. The flame front begins to develop near the spark plug electrode. Note the piston is before TDC

3. The flame front moves across the combustion chamber. The piston is at TDC

4. The flame front reaches the other side of the combustion chamber and the piston is moving down.

 

The speed of the flame from can be calculated by the engine design boffins, with the speed of the flame front being one of the variables in the calculation.

 

So, aero-engines have two spark plugs per cylinder for:

1. Even combustion. With spark plugs on opposite sides of the combustion chamber, two flame fronts are generated and there is a quicker and more complete burn of the fuel/air mixture.
   
2. More power. The combustion of the fuel/air mixture produces pressure which acts on the crown of the piston. Pressure is a force. The application of this force to the piston changes the Momentum of the piston The speed with which the momentum of an object changes is called Impluse. The speed at which the fuel/air mixture burning increases the pressure in the combustion chamber affects the speed at which the force from the pressure increases the momentum of the piston, which then becomes the torque force applied to the crankshaft, which ultimately is the source of the engine's horse-power. Therefore two spark plugs mean faster burn, hence more power.
   
3. Reliability. A bonus arising from having two spark plugs per cylinder is redundancy. If one magneto, or HT lead, or spark plug fails, there is still one to carry on. However, if you lose the benefits described above, you lose power. If you are flying with a 4-cylinder engine, you could lose up to 25% of the rated power of the engine if one spark generating system fails.
   

So why do you get at rev drop when you go from "Both" to one?

1. Slower burn of the fuel/air mixture
   
2. A lower rate of change of momentum of the piston, resulting in the generation of less torque at the crankshaft. Less torque = less horse power.
   
3. Less horse power means a slower turning crankshaft, indicated by lower revs.
   

 

Note: The stills were taken from a gif file that was too large to load. Each still is about 250 -300 Kb, so it you want the full set of 18 stills, or the gif, PM me with your email address and I'll send what you want.

I would say 3 Reliability is the only reason. There is no power advantage over a centrally located plug. Ignition timing takes into account the speed of combustion. I can think of only one twin spark car engine, it was in the Nissan gazelle, if it was such a great idea every manufacturer would have done it. With 4 valves per cylinder a single plug is the only option.

With 2 plugs you use less advance, With a single plug you use more. Once adjusted for there's no significant difference in power output. The 2 spark Nissan was probably to meet emissions more than anything else.. Most of the Burning should be before TDC. Some poorly designed racing motors ran over 45 degrees advance in pre war WW2 times .Side valves are very bad in flame propagation as well as having a high area/volume ratio and you can't get a reasonable compression ratio without affecting gas flow. There's also more uneven cylinder heating causing bore distortion.

A central plug needs less advance. Almost universal on a 4 valve layout same as with 2 plugs . Reason ...less distance for the flame to travel. The Jabiru should have less RPM drop than those with a plug each side of the head because the plugs are together...Nev

Edited April 13, 2020 by facthunter

Several people answered the wrong question (why 2 plugs) not what was asked, why the RPM drop. For my money it is due to incomplete combustion (3). Which is why on low rpm engines like Jabiru the drop is too small to notice. Everything happens at half the speed on a Jab compared with a 4 stroke Rotax, everything except the rate of burn progression. So you still get almost complete combustion on low rev engines and little or no drop of RPM.

 

Mike

Several people answered the wrong question (why 2 plugs) not what was asked, why the RPM drop. For my money it is due to incomplete combustion (3). Which is why on low rpm engines like Jabiru the drop is too small to notice. Everything happens at half the speed on a Jab compared with a 4 stroke Rotax, everything except the rate of burn progression. So you still get almost complete combustion on low rev engines and little or no drop of RPM.

 

Mike

 

Ok I see that I responded to the statement and not the question. On a jabiru the drop would be less than say a lycoming because the plugs are both on the same side of the combustion chamber. By the time the flame front reaches the far side all the mixture will be consumed around the plug side regardless of one or both plugs firing.

Edited April 18, 2020 by Thruster88

The speed of the flame from can be calculated by the engine design boffins

The speed of the flame can't exceed the speed of sound without damaging the engine. The speed of the flame is directly related to the length of the shortest hydrocarbon chain in the fuel. Available oxygen plays a part but not as much as the fuel quality.

 

Doesn't answer the original question, but there is often no calculation here only observation of the pressure peak using a sensor

https://en.m.wikipedia.org/wiki/Laminar_flame_speed

 

From the 1920s. Now on the bookshelf next to the table of red ball densities

From 1983

https://www.sciencedirect.com/topics/engineering/flame-front

With my Jab 3300A there is a noticeable RPM drop at idle but it is imperceptible at 2000 RPM unless there is a dodgy plug.

Interesting Jabiru specific no more than 100rpm drop from 2000rpm. That’s 5%. With both plugs being together it’s likely the difference in efficiency of the complete electrics from spark plug backward. If somebody was really bored they could swap components 1 at a time and take notes to determine which particular parts or combination of parts affected the initial 5% dropped side.

 

Based on the Jabiru being plugs together and also potentially dropping you’d then do the same on a Rotax and check if the same thing happened and swapping parts changed the side with the drop.

 

You’d then match parts that have no drop on either side so efficiency of circuit is eliminated. Then flick a side off and see what happens. Then compare plugs together to plugs apart.

 

I think you’d need to be really bored to do it.

 

edited just now..... I see the post with Jab engine identical. Makes the spaced plugs likely more efficient.

I recall reading a discussion about the inadvisability of having both plugs firing exactly together.

Something about colliding flame fronts and detonation. As a result, people were advising a slight delay between the two.

From a quick observation.

Two plugs together has a single flame front.

Two plugs on oposite sides has twice the fronts, giving half the time to cover the same distance.

On the Lyc with one off, the only plug has to travel twice that distance.

Hopefully makes a little bit of sence. LoL

spacesailor

I recall reading a discussion about the inadvisability of having both plugs firing exactly together.

Something about colliding flame fronts and detonation. As a result, people were advising a slight delay between the two.

 

If that was a thing lycoming would know about it by now. I was taught to time both mags to the one and only timing mark at 25btdc.

There's a difference between "detonation" and flame front intersection.

 

During a normal combustion cycle, the flame front expands from the point of ignition out towards the cylinder walls and piston crown, this burn process can take up to 90 degrees of crankshaft rotation to completely burn. Detonation is identified as any spontaneous combustion occurring after the burn process has already begun, and is independent of the initial flame front. This uncontrolled event can originate from anywhere within the chamber and is usually caused by high cylinder temperatures or an incandescent ash deposit.

 

High temperatures can be caused by a over lean mixture where there is more than enough oxygen in the fuel/air mixture for the fuel to burn too rapidly

Air-fuel ratio (AF or AFR) is the ratio between the mass of air ma and mass fuel mf, used by the engine when running:

 

The ideal (theoretical) air-fuel ratio, for a complete combustion, is called stoichiometric air-fuel ratio. For a gasoline (petrol) engine, the stoichiometric air-fuel ratio is around 14.7:1. The combustion is possible even is the AFR is different than stoichiometric. For the combustion process to take place in a petrol engine, the minimum AFR is around 6:1 and the maximum can go up to 20:1. When the air-fuel ratio is higher than the stoichiometric ratio, the air-fuel mixture is called lean. When the air-fuel ratio is lower than the stoichiometric ratio, the air-fuel mixture is called rich. For a perol engine, an AFR of 16.5:1 is lean and 13.7:1 is rich.

 

The AFR affects the torque an engine can produce. Example of air-fuel ratio (AFR) function of engine speed and torque for a car engine, but the principle is the same for aero engines.

Mixture has a lot to do with how easily it lights up from the ignition source. Drops of fuel don't count so if idle fuel or with the engine being cold, it needs to be enriched till the engine gets warmer and the fuel vapourises better. Poor vapourisation also retards the flame propagation rate giving worse response than when warmer up fully.

You only check for a dead CUT with motor at idle. It's Pretty useless for any other check. The 1500 plus rpm or such is so you are way from any retarding effect.. Impulse where fitted will retard at lower revs . That's why you start on the mag with impulse if you value your starter drive. and your hand if you are prop swinging. (armstrong starter) IF you operate with a plug out, that cylinder will effectively have about 10 degrees retard effect due to the longer flame travel if the plugs are wide apart, so it's not doing the motor much good. Nev

Two plugs for redundancy?

I think you will find that the designers aim to have maximum pressure in the cylinder somewhere around 17 deg after TDC, to get the most power. If you have egt gauges you should see an increase in egt when you do the mag check, due to a slower burn and therefore more hot gas into the exhaust.

While it might be anti intuitive it's best to have all the burn complete before the piston starts going down. Different to a steam engine where you keep letting steam in. As soon as the compressed volume starts to increase the temp and pressure drop pretty quickly.. Some earlier racing engines had 50 degrees of advance pre war with hemispherical combustion chambers and high top pistons. This gave a combustion chamber shape resembling an orange peel and very poor flame propagation. No squish areas and poor turbulence also with crude port shapes..Nev

Great description of why the RPM drop OME. Spot on.

Now to why airplanes have two spark plugs and cars have only one. Airplane engines are over-square. Their bore is wider than the length of the stroke. They need two separated spark plugs to start two flame fronts, in order to get a complete burn before the piston reaches TDC. (You're right Facthunter)

I really thought all pilots already knew this, but the reason airplane engines are over-square is to keep the weight of the crankshaft down. A crankshaft with a shorter stroke can be made significantly lighter. The further that the big end journals are out from the main bearings the beefier that the metal must be, to carry the torque around the cranks from one end of the engine to the other. By far the heaviest single component of an engine is the crankshaft. It is well worth the extra weight and complexity of dual ignition, for the significant saving in crankshaft weight. As an added bonus we get some redundancy (at reduced power) in the event that one ignition system fails. Although this was never the primary reason for dual ignition. But all you guys already knew that . Didn't you?

Edited April 19, 2020 by Snoopy

Some car engines are over square also, not limited to just A/C engines.

Some car engines are over square also, not limited to just A/C engines.

I thought most modern engines had gone over square for efficiency reasons.

Most motorcycles have very short stroke and very high revs. Suzuki GSX-Rs don't rev, they vibrate!

You only check for a dead CUT with motor at idle.

This I didn’t know. It’s nice to have in the tool box of checks. Almost positive but wouldn’t bet money it wasn’t in King School, either FAA book I read from cover to cover or in Cessna handbook.

Poor vapourisation also retards the flame propagation rate giving worse response than when warmer up fully.

Expect we are all looking forward to the time when the word “vaporisation’ doesn’t take you straight to coughing and Covid 19

The Sainty V8 vibrates too, ,now running 3 seconds.

spacesailor

An old mature aero engineer once told me that dual mags were needed in early days because they were so unreliable that operating on one was quite common hence the reason for a spare ignition source. But all reasons given so far make sense

An old mature aero engineer once told me that dual mags were needed in early days because they were so unreliable that operating on one was quite common hence the reason for a spare ignition source. But all reasons given so far make sense

100% still the number 1 reason would make sense. I’m wondering if anyone has a story of having to switch after take off to avert an emergency landing.

 

Second reason in the case of the spaced spark plug engines you’d think would have to be a say 5% increase in efficiency. That would add up in commercial flying.

 

I’m now wondering if the Jabiru engine designers slapped themselves across the head at any stage and went “that’s why they put them on opposite sides” “Bugger, I better make up a good sounding engineering reason why mine are on one side"