4 Cylinder Inline Engine

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Hi Ray,
I worked in a car company - the guy who did the inlet manifold work sat next to me... a carb into a central Plenum. from which into the mid-point of a traditional "1 long pipe with 4 branches" He laboured for months, with supporting visitors from the main design group, and they "tweaked" the shapes, grooves, little air-guide baffles and shapes inside the castings to "encourage" fuel to follow the air-flow turning into the inner cylinders, instead of going straight-on to the outer cylinders! To get it right at full throttle meant it would never be right at anything less.
It is no wonder the fuel injector per cylinder made such a huge improvement to power across the whole range, smoothness, good low emissions (with positive feedback constantly tweaking fuelling), etc.
Carbs are Complex, Highly Sophisticated crude devices, that then dump a fuel-air mix into a single manifold that proceeds to separate the fuel droplets from the air so each cylinder gets a different mixture! Air and vapour go around corners, but droplets carry-on....
The Manifold "flow expert" explained all the problems to a group of us and "why" it could never be right for all engine conditions, just a crude average at any point... - but generally at its best for max power (necessarily). - Unless "fuel economy around town" was the sales USP... in which case a different version tuned for that condition was used. - Depended on the "legislation test" for the market really. - And everyone got the optimum version, so all markets just had one manifold.
It was no wonder the Racing guys could always get a better manifold for racing! - Or used 4 carbs for 4 cylinders!
GOOD LUCK tuning your design. 4 "equal" pipes from the plenum should work...
A guy at the local Model Engineering club ended up - after 3 previous manifolds - using a plenum to 2 equal curved pipes, each then dividing into 2 pipes to 1 & 2, and 3 & 4, for a 1,3,4,2 engine. But that wasn't perfect, it just "ran OK". So each cylinder "saw" the same set of curves and lengths from plenum to cylinder head. - Just "opposite hands", which didn't vary the intake flow.
Some early radial engines brute forced the problem by ducting the mixture through a rotating fan mounted on the rear of the crankshaft, the spinning blades 'stirred' the mixture in an attempt to even out the fuel flow to each cylinder. Later that role was assumed by the gear driven supercharger.

A bit of a random thought: what happens if you have your carb feed a single large plenum, with a 'trumpet' for each cylinder inside? I know this layout is popular on fuel injected racing engines but would it work at all with a carb?
Hi N1000.
This is what the more modern manifolds do, but with multi-point fuel injection.
For carbs, there is an issue for gas dynamics. Really, you need to minimise the air volume between cylinder (intake valve) and venturi (choke/throat) in the carb. (At the jet).
An example:
If the cylinder volume is 2 cu.in. and manifold volume 1/2 cu. in. Then the pressure when the cylinder has tried to suck-in 1 stroke of air, is sucking a volume of 2 1/2 cu.in. so can get down to 20% of atmospheric pressure. So the air whizzes through the darb, making it easy to tune against a "good suck" at the jet.
But if the 2 cu.in. cylinder met a 2 cu.in. manifold, the the best the cylinder could suck would be 50% of atmospheric pressure. So less suck and less easy to tune the carb.
Not impossible, but less easy. You want the carb to work from low to high speed of the engine, which actually varies the suck, and from idle to wide-open-throttle. "WOT".
This is a crude model oversimplification of how "manifold volume" affects suck at the carb.
Really it is all dynamic, and the carb works with a sort of DC flow with a big AC ripple on top. The venturi pressure varies as V-squared, so peak velocity is everything when tuning carbs.
A big plenum reduces the ripple and hence the peaks.
Hope that crudely helps?
If engine is not intended for performance but for smooth running, this might be of help (sometimes hot spot is bath shaped and diverts air flow 90 deg. to retain and vaporise gas droplets). But this is old technology, mating with less-than-perfect carbs.:


Rochester Carburetors by Roe, page 26
"Intake Manifolds -- Fuel distribution is affected by exhaust-heated hot spots in the manifold just under the carburetor. ... If heat isn't used, the engine will run rough and distribution problems will increase"


"This is a very interesting question. Hot-spots or heated inlet manifolds are another example where an engine does not behave as one may expect.

Thinking about the 3.3L engine. During one full revolution 3 cylinders draw in mixture, i.e. 1.65L. This has to travel through the inlet manifold. Assuming the approximate diameter of the inlet manifold is 50mm (2”) then at 2,500 rpm, the mixture will take approximately 8ms (0.008s) to travel down the longest 300mm runing length. This is far too short a time for heat to be transferred from the hot-spot to the bulk of the mixture.

A heated manifold or hot-spot has virtually no effect on the temperature of the mixture in a running engine. So why are they there?

Engines can suffer from a problem called “pooling” where liquid petrol collects in the inlet manifold. Its flow into the engine is uncontrolled. When it does, it makes the mixture temporarily richer and causes uneven running. Pooling is worse when an engine has just been started or is running cold on the choke. To address this, the carburettors on the XPAG are semi-downdraft. The carburettors and inlet manifold slope downwards towards the engine to allow the petrol to drain into the cylinder. The hotspots or heated exhaust are an alternative way of addressing this problem. They vapourise any liquid petrol as it collects in the inlet manifold."
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What I have said above comes to add to K2's details. Uneven mixture is both effect of suspended fine fuel particles in air flow that has a rhythmic beat on cylinders and a continuous flow of a film coming from larger fuel particles which adhere to manifold walls. Both have higher inertia and hardly follow the hands of the "drummer", like air/vapours do.
From what I have seen on my engines since the pipes to each cylinder is transparent there is an unequal amount of fuel going to the weak cylinder. I hope the current setup will help with this.

Made a base from poplar with a walnut stain and clear finish today.

Also started on a separate box for the electronics.
I caught myself using a caliper to measure the wood pieces. :rolleyes:

Thanks for looking
My thinking on the plenum/trumpets layout is that the trumpets could act as standpipes that stop the 'pooling' from entering the cylinders at all. Though perhaps this might lead to problems with the mixture ratio when the throttle is suddenly opened or closed.
Nerd1000 like many of the things I try I have no idea if this will help or hinder but just trying something different.

One of my sides on the wood base was 0.025 shorter than the opposite side and I was trying to think of a way to even it up when it struck me that it doesn't matter.

I for one am looking forward to seeing what the results are like!
Hi Ray,
Just something from my "Car maker" experience. The 2 inner manifold pipes would normally come from further away - not the nearest - holes in the plenum. Makes the inlet tracts nearer to the same length as those to the outer cylinders. In fact a lot of work went into making the inlet tracts the SAME length to eliminate differences in "resonances": I.E. 2 outer cylinders with longer tracts could at some resonant point in the rev range take ALL the mixture effectively, leaving a transient depression (or pressure!) for when the next inlet valve opened, which if an inner cylinder with shorter tract length was not resonating and unable to cope with the different manifold pressure... so not getting the right mixture and causing a flat spot at the resonant engine speed... instead of the expected "boost".
Simply, when all the inlet tracts are the same length there is only one "potentially problematic" resonant speed to manage especially - instead of 2, when there are 2 sets of inlet tract length.
Longer tracts also resonate at a lower speed, - usually within an engine's rev range, but shorter tracts sometimes resonate above the max engine speed.
Racers use resonances to get extra power. The "common man" driving his mass produced car doesn't want to feel a resonance that can give a sudden surge of power, but sometimes these are tuned to suit other factors. E.g. if an engine is on an automatic transmission that normally runs the engine at a fixed rpm (max. torque rpm). then the inlet tract length (and maybe exhaust lengths) will be tuned to resonate at that length to get the natural power boost available.
For a "bench idling engine" it matters not a jot - except to passing "rivet counters".
I like the clear pipes? What material have you used? - My supplier doesn't do clear metal...:D
Getting very close to firing it up ! Can't wait ! Fantastic job.
Hello Ray,
I really like the updraft carburetor idea along with the fuel hose used as intake runners. Very clean looking four cylinder. Eager to see it fire up soon!
Thanks Sparky and raveney I'm getting excited too. There are so many things on this build that are new to me and unproven I just don't know what will happen.

I have other obligations tomorrow but maybe Monday.

Keep on. Lots to enjoy with this project. Innovative using clear plastic intake tubes.
Maybe it needs and indicator light so when ignition is switched ON and engine not running you can see that easily? Seems standard on some of my colleagues' models. It meant at shows when their engines stopped (usually a shortage of fuel) I could see the ignition was still ON and switch OFF to save batteries and electronics.
And will it have a push-button starter later? One guy used a sprag clutch from a battery model car for the belt drive from starter motor to crankshaft, as it disengaged when the starter was not powered.
Hope the engine doesn't overheat...?
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No starter or indicator light K2 I hope to be able to hand start the engine when warmed up.

Well I did get some time today and put the gas tank together with some 5min epoxy. I also made a tank stand and installed both on the engine.
And a little closer up picture.

On all of my other engines that use this type of carburetor I have installed a throttle lever. The lever is either hard to move or moves by itself so there won't be one on this engine. I installed a brass weight on the carb lever to hold it against the throttle stop at the idle position. I also added a brass knob to the throttle stop screw that is easy to grip and turn. When starting the engine I will turn the idle stop screw in 1 turn first and then start the engine. I will then back out the screw to set the idle speed desired and the high speed is still available by lifting the weight with your finger.
That's the way I hope it will work.

Thanks for looking
Think you'll find those tygon tubes will flutter and close up to a degree on running.

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