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Author Topic: Engine design and modifications Q/A thread  (Read 2493 times)

AussieDave

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Re: Engine design and modifications Q/A thread
« Reply #15 on: October 23, 2013, 09:55:44 AM »
Tom, I think maybe it's time to dig out a few of those instructional threads from a few years ago. Y'know, the ones that began "Let's talk about...". Those were great.
+1
 I read most of your Articles on the yahoo page( awful to navigate by the way) and I reckon ye should publish them in a chapter format somehow.... Didn't jump in right away coz well to many posts makes Aussiedave a PITA... But I was wondering about mixture flow as it leaves the injector manifold into the inlet chamber ( bowl?) does the bowl act as a Venturi type affair and speed up the flow as it reduces volume to the inlet port or does it act to slow the flow down and increase the static pressure? Or should it try to maintain the existing velocity? Also previous posts have mentioned that the length of the efi manifold functioned to decrease turbulence- I think some one said their mileage decreased when they put pods directly on the injector. But isn't turbulence a good thing - keeping the fuel in suspension and helping an efficient burn?
« Last Edit: October 23, 2013, 10:26:33 AM by AussieDave »
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ace.cafe

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Re: Engine design and modifications Q/A thread
« Reply #16 on: October 23, 2013, 10:52:08 AM »
RGT,

Yes, that is a viable option.  The Ace Shotgun Rockers have lower ratio,  but they will have some of that effect.
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ace.cafe

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Re: Engine design and modifications Q/A thread
« Reply #17 on: October 23, 2013, 01:55:43 PM »
+1
 I read most of your Articles on the yahoo page( awful to navigate by the way) and I reckon ye should publish them in a chapter format somehow.... Didn't jump in right away coz well to many posts makes Aussiedave a PITA... But I was wondering about mixture flow as it leaves the injector manifold into the inlet chamber ( bowl?) does the bowl act as a Venturi type affair and speed up the flow as it reduces volume to the inlet port or does it act to slow the flow down and increase the static pressure? Or should it try to maintain the existing velocity? Also previous posts have mentioned that the length of the efi manifold functioned to decrease turbulence- I think some one said their mileage decreased when they put pods directly on the injector. But isn't turbulence a good thing - keeping the fuel in suspension and helping an efficient burn?

Okay Dave, I'll take these things in order.
First, your terminology of "inlet chamber(bowl)" is probably more accurately termed "inlet port", and there is a "bowl" there at the valve end of the inlet port, where the port turns a bit and opens up behind the valve itself.

The injector nozzle in the UCE is essentially a port injector, and it is aimed into the inlet port at an angle that is as close to the back of the inlet valve as they can make it. The object of this is to have the least reliance on the air flow for atomization and delivery of the fuel. The fuel is sprayed in a very fine mist at the last moment as the valve opens, so that it is carried in by the air immediately when the valve is open. This helps to reduce fuel drop-out and puddling, in comparison to what is often seen in carbureted engines. Injected engines generally do better at delivering the fuel in a better atomized state as it enters the cylinder.

This question about whether the port is acting as a venturi for speeding up or slowing down the mixture, or keeping it at the same speed is dependent on the design of the port, which can vary greatly on different engine designs. In the Bullet, and our Ace ports, the shape turns and expands into the bowl as it approaches the valve, with the purpose of slowing the flow and increasing the pressure recovery there, so the air can better make the turn, and become partially pressure-recovered as it will then make its way past the valve into the combustion chamber where it will recover more pressure and then move down into the cylinder itself. During all of this pressure recovery phase, it is slowing down and trading velocity energy for pressure energy, because energy must be conserved. If it does not recover pressure properly, it will become turbulent and produce vortices and eddies and find all possible ways to dissipate the energy in that turbulence. When it does that, the flow energy is released into this chaotic mess, and the speed plummets, and pressure is slow to recover, and the turbulence creates blockage and impediments to the air stream coming behind it. These are flow losses, but they come after the valve. Flow losses after the valve comprise about 50% of all the flow losses in the system, but we have very little methods to control these losses, and just concentrated on what we could control with the port and valve. It's only recently that these flow losses after the valve are being addressed, and we use methods with our porting to address this to reduce some of our flow losses in our heads.
The purpose of the port is to speed up the air in a venturi as it travels from the outside atmosphere into the port, trading the atmospheric air pressure for air stream velocity, and then as it gets near the cylinder, trade off the velocity for pressure recovery in a controlled way, to get the air back into its atmospheric-pressure condition in the cylinder with the least loss of flow energy. Ideally, you want it to behave as close to a Bernoulli lab experiment venturi as possible, like we see the tube with the narrow waist in the middle that has gradual narrowing and expanding on each side of the narrow portion. Of course, it never gets anywhere near that good, but that is the ideal which we are trying to simulate with our efforts. The shapes and sizes and methods use will vary, based on the challenges of the physical restraints of the engine shape and intention of the design, so not all ports are designed the same.

Regarding the turbulence in the UCE throttle body, it seems that this turbulence influences/confuses the mass-air pressure sensor, and thus has a negative influence on the accuracy of way the ECU delivers its fuel charge to the injector. That system seems to respond better to a smoother air flow, which is typically served by a still-air column or chamber prior to entering the throttle body, so that the sensor gets a better reading.
Turbulence is a fact of life, and we try to minimize it in most cases, but in other circumstances it can be utilized in small and local ways to do some jobs for us, as long as we keep it under control to do only/mostly what we want it to do.

This is a really big subject on fluid dynamics which could have MUCH more detail, but I hope that I got the highlights across in an understandable way with the brief discussion above.
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AussieDave

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Re: Engine design and modifications Q/A thread
« Reply #18 on: October 23, 2013, 10:23:17 PM »
 Yes , thanks Ace , that helps my understanding muchly. I find it remarkable that you are able to create a positive pressure in the cylinder with your raised port design, just using induction and the momentum of the air! My understanding is that most engines still have a slight vacume at the end of the induction stroke- that is , they don't fill completely to atpospheric pressure. It certainly is a tricky environment for air flow and I find it fascinating. Wish I could see your flow bench in action. I'm curious as to how you model the changes you make-are there simulations you can run try different shapes in the passages? I hope I'm not encroaching on your trade secrets:)
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ace.cafe

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Re: Engine design and modifications Q/A thread
« Reply #19 on: October 23, 2013, 11:00:12 PM »
Yes , thanks Ace , that helps my understanding muchly. I find it remarkable that you are able to create a positive pressure in the cylinder with your raised port design, just using induction and the momentum of the air! My understanding is that most engines still have a slight vacume at the end of the induction stroke- that is , they don't fill completely to atpospheric pressure. It certainly is a tricky environment for air flow and I find it fascinating. Wish I could see your flow bench in action. I'm curious as to how you model the changes you make-are there simulations you can run try different shapes in the passages? I hope I'm not encroaching on your trade secrets:)

Dave,
Typically it requires wave tuning in the inlet and exhaust tracts to achieve over 100% volumetric efficiency.  And it doesn't always happen with the first attempts.
Essentially, the techniques involve setting appropriate tube lengths which work at certain harmonic frequencies with the targeted engine rpms to cause extraction waves in the exhaust pipe and ramming waves in the inlet tract. The extraction waves in the exhaust tract are timed to scavenge the last remaining gases in the combustion chamber so it is completely empty, and also create a suction to assist the inlet mixture to begin coming in prior to the piston descending on its intake stroke. The ramming wave in the inlet tract is timed to provide a positive pressure wave just prior to the closing of the inlet valve, to help push in some extra mixture at the last moment and prevent any reversion back up the inlet port from the building cylinder pressure created by the ascending piston. It's a very timing-sensitive matter, and is only effective at a certain bandwidth of rpms where it is intended to operate.
« Last Edit: October 23, 2013, 11:04:20 PM by ace.cafe »
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AussieDave

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Re: Engine design and modifications Q/A thread
« Reply #20 on: October 23, 2013, 11:09:15 PM »
Ok- wave tuning. Will now go read all I can find! Thanks!
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singhg5

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Re: Engine design and modifications Q/A thread
« Reply #21 on: October 28, 2013, 04:10:20 PM »
This is a general question on engines, related to kick start gear.

The electric starter motor gears are connected to crankshaft through sprag clutch mechanism that transfer power when required. I guess there is no such sprag clutch mechanism for the kick start gear.

How does kick start gear engage and disengage from the transmission shaft that it rotates to start an engine ? Do you have any pictures to clearly see how it is connected in RE ?
   
« Last Edit: October 28, 2013, 04:20:39 PM by singhg5 »
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ace.cafe

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Re: Engine design and modifications Q/A thread
« Reply #22 on: October 28, 2013, 04:30:29 PM »
This is a general question on engines, related to kick start gear.

The electric starter motor gears are connected to crankshaft through sprag clutch mechanism that transfer power when required. I guess there is no such sprag clutch mechanism for the kick start gear.

How does kick start gear engage and disengage from the transmission shaft that it rotates to start an engine ? Do you have any pictures to clearly see how it is connected in RE ?
 
It is a simple ratchet/pawl system. It is not engaged during running, like the sprag. A sprag ts sort of like a ratchet mechanism too, but it stays engaged with the engine gears all the time, and so it is vulnerable.

I am posting from my phone right now, so I can't put any photos up. The kick start ratchet and pawl parts can be seen in the exploded diagram in the parts book.

Also,  these kick start pawls and springs sometimes break too, but it is cheap and easy to fix it.

I personally think that a "bendix" type of electric starter, like cars have, would be a better choice than a sprag starter system.
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singhg5

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Re: Engine design and modifications Q/A thread
« Reply #23 on: October 28, 2013, 04:46:54 PM »
It is a simple ratchet/pawl system. It is not engaged during running, like the sprag. A sprag ts sort of like a ratchet mechanism too, but it stays engaged with the engine gears all the time, and so it is vulnerable.

I am posting from my phone right now, so I can't put any photos up. The kick start ratchet and pawl parts can be seen in the exploded diagram in the parts book.

Also,  these kick start pawls and springs sometimes break too, but it is cheap and easy to fix it.

I personally think that a "bendix" type of electric starter, like cars have, would be a better choice than a sprag starter system.

So the pawl can break on a kick back, if the kick lever is held down and not allowed to return !

That would require splitting the crankcase, isn't it ?
« Last Edit: October 28, 2013, 04:55:51 PM by singhg5 »
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ace.cafe

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Re: Engine design and modifications Q/A thread
« Reply #24 on: October 28, 2013, 05:15:07 PM »
So the pawl can break on a kick back, if the kick lever is held down and not allowed to return !

That would require splitting the crankcase, isn't it ?
No,  at the bottom of the kick stroke it disengages as it moves past the last ratchet tooth. Then it can safely return to the top, propelled by the return spring, with the pawl in the free direction.

 The typical breakages would be a kick back in mid-stroke, or the user trying to repeatedly kick the engine thru the compression stroke without using a decompressor, which over-stresses the pawl with too much load.

 In the Iron Barrel engine bikes, the kick start ratchet/pawl/return spring are easily accessible under the outer transmission cover for roadside servicing, if necessary. If it is inside the engine case on the UCE, then that would be much more difficult to service a broken pawl or return spring. I would hope that they would be behind an engine cover, and not inside the engine case halves.
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Re: Engine design and modifications Q/A thread
« Reply #25 on: October 28, 2013, 09:48:47 PM »
The Service Manual seems to show that the kick starter spring can be replaced by removing the right side cover and then the small cover that the kick starter shaft comes out of.
To do anything to the ratchet or the gear it drives, the engine case must be split and the transmission gears removed.  Only then will the kick starter gear/ratchet be able to come out of the engine. :(
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singhg5

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Re: Engine design and modifications Q/A thread
« Reply #26 on: November 01, 2013, 05:26:28 PM »
Tom:

Can you discuss the use of two smaller size intake/exhaust valve set vs one larger valve for intake/exhaust in an engine ?

How do you decide when to use 2 valve-set and when to stick to 1 valve-set ?
« Last Edit: November 01, 2013, 06:30:07 PM by singhg5 »
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ace.cafe

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Re: Engine design and modifications Q/A thread
« Reply #27 on: November 01, 2013, 06:46:00 PM »
Tom:

Can you discuss on the use of two smaller size intake/exhaust valve set vs one larger size valve for intake/exhaust in an engine ?

How do you decide when to use 2 valve-set and when to stick to 1 valve-set ?

Singh,
The common practice today is to use 4 valves per cylinder(2 intake, 2 exhaust). This provides more valve area for a given bore size.  Two valves for the intake or exhaust also have lighter mass per valve, so revving higher is easier to do without the valves getting out of control.

Generally speaking, pushrod engine designs will be 2-valve, and overhead cam engines could be either 2-valve or 4 valve.

The decision would be based on the need of the engine in its normal role. If the job can be done with one intake and one exhaust valve, it's cheaper to produce. This would be a relatively low revving design, like the Bullet.
For high revving short stroke screamers,  they need to operate at high rpms where low mass and high flow rates are very important.
« Last Edit: November 01, 2013, 11:57:40 PM by ace.cafe »
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High On Octane

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Re: Engine design and modifications Q/A thread
« Reply #28 on: November 02, 2013, 01:48:24 AM »
Tom - On my '59 Chief that I'm building for Bonneville I've decided for my initial build I'm going to go high compression as opposed to a turbo set up.  I want to build either 13:1 or 14:1 compression ratio pistons and run on methanol.  I'm also going to use the 1 piece alloy Interceptor cylinder/s from Hitchcock's to help create clearance for the dome top pistons and help keep the motor cool.  Keeping in mind I'm sending my cams out to have them custom ground to whatever profile I want/need (basically as big of a lobe I can fit inside the crankcase), how far can I retard phase the intake camshaft to achieve a 7500-8500RPM redline?  My thoughts are to still build the cams as radical as possible with a pretty large duration to flow in/out the high compression charged cylinders, but phase the cams with an ideal overlap between 4000-7500RPMs.  Essentially full race style set up with the idle set at 1500RPMs and sounding like it wants to die, snapping and spitting, but then when you hit the throttle it rattles your bones loose.  Thanks.   :)

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ace.cafe

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Re: Engine design and modifications Q/A thread
« Reply #29 on: November 02, 2013, 03:00:16 AM »
You can close the intake valve as late as 90 degrees ABDC.
The Fireball closes the intake at 78 degrees ABDC.
« Last Edit: November 02, 2013, 03:04:32 AM by ace.cafe »
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