Exhaust and Intake Valve Diameters

MolaKule

MolaKule

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In terms of Gas Dynamics (such as flow velocity, thermodynamics, piston crank angle, and pressures), explain why the Intake valve has a larger diameter than the Exhaust valve.

Assume a naturally aspirated, single-cylinder, spark ignition engine.

Rule number 1. We will not criticise each other's answers since we will deal with clarifications later.
 
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redbone3 said:
The intake valve operates under a lower pressure than the exhaust. The exhaust must pass a larger volume of gas than the intake.
Click to expand...
Not sure I could do better than this. The intake stroke seems to make only about 1 atmosphere of draw, assuming fully open throttle valve, as the piston descends. But the exhaust is under much greater pressure, especially as the piston ascends, pushing exhaust outward, as the combustion gasses are still expanding at even the end of the compression stroke. If PV=nRT, then the much larger pressures must mean that smaller passageways are going to have similar rates of mass flow (not volume, as exhausts is larger due to the increase in heat)... the higher pressure can overcome a smaller volume of the port, and similar rates of flow (mass) can occur.

I've always marveled at how well it worked--on the draw, the intake valve is a bit of a restriction, but has the angles kinda working for it--but during exhaust, any exhaust has to take more wild bends around the valve.

Beyond that, I'm not mechanical engineer, so that's about as technical as I could get.
 
To me, the intake volume is inherently smaller as it is cooler, you need volume to pack the cylinder with air and fuel so you need a large valve. The exhaust volume is bigger due to heat, and so its velocity is higher when the exhaust valve opens. You only have so much room in the cylinder head, so you want a bigger intake valve.
 
redbone3 said:
The intake valve operates under a lower pressure than the exhaust.
Click to expand...
This is true. The best the natural atmosphere can provide is the approximate 14.7 psi of push (ignoring the effects of barometric weather changes) into the cylinder. However the piston can push much harder during the exhaust stroke; it generates a higher delta-P. Hence, the exhaust valve doesn't need as much circumferential area to pass the same volume in the given time allowed by the valve timing. We are assuming valve timing is equal in both events ... Additionally, the pressure from combustion generates a much higher pressure which exists until the very late stage of crank angle as TDC approaches.

However, there is no change in the "volume" of gas (matter) in the fuel/air charge from intake to exhaust. You cannot destroy matter; you can only change its phase-state. We also have to admit a small amount of the matter does escape past the rings and into the crankcase, and therefore doesn't have to head out through the exhaust valve; that's a tiny fraction in the grand scheme of things. Let's also acknowledge that in some engines, a small amount of trapped exhaust is part of the emissions control protocol. But now we're getting into the weeds. For the sake of simple conversation, there's no change in volume; what goes in must go out.



That's the way I see it.



.
 
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Intake flow is from vacuum and exhaust is moved by the piston rising in the bore and depending on the cam timing a varied amount of combustion pressure.
 
Also, there is only so much room to fit the valves. The engineers had to decide how to divide up the space. As described in the previous posts, the intake valves need more ot the available flow area.
 
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Here are the details on my 6.0 LY6. At one time a 2.02 “ intake valve would have been considered a performance item on a SBC.

6D5CF344-E14B-499D-9A85-3AD7097DDF8A.webp
 
dnewton3 said:
This is true. The best the natural atmosphere can provide is the approximate 14.7 psi of push (ignoring the effects of barometric weather changes) into the cylinder. However the piston can push much harder during the exhaust stroke; it generates a higher delta-P. Hence, the exhaust valve doesn't need as much circumferential area to pass the same volume in the given time allowed by the valve timing. We are assuming valve timing is equal in both events ... Additionally, the pressure from combustion generates a much higher pressure which exists until the very late stage of crank angle as TDC approaches.

However, there is no change in the "volume" of gas (matter) in the fuel/air charge from intake to exhaust. You cannot destroy matter; you can only change its phase-state. We also have to admit a small amount of the matter does escape past the rings and into the crankcase, and therefore doesn't have to head out through the exhaust valve; that's a tiny fraction in the grand scheme of things. Let's also acknowledge that in some engines, a small amount of trapped exhaust is part of the emissions control protocol. But now we're getting into the weeds. For the sake of simple conversation, there's no change in volume; what goes in must go out.



That's the way I see it.



.
Click to expand...

can't talk about volume without pressure in case of gasses. The volume trying to expand is the reason the pressure increases and engine produces power. It will certainly expand when the chamber isn't closed anymore.
 
In racing engines without valve diameter limitations, you fit the largest intake valve you can that won't shroud (excessively) or have clearance issues and the exhaust gets whatever is left.

On the intake stroke, you have ~14 psi in the intake filling a cylinder that's 3-4 psi lower (at most). On the exhaust side, there's 100+ psi residual combustion pressure in the cylinder when the exhaust valve opens. The engine's fart is screaming out of that cylinder in blowdown with the rest being pushed out by the piston or scavenged during overlap. Thus, you don't need as much valve curtain area on the exhaust side. The smaller exhaust valve also helps prevent reversion.

Another reason, more durability than power or efficiency, is the ability to open the exhaust valve against cylinder pressure. A 1.60" valve has a surface square area of 2.01 in^2. With 100 psi residual combustion pressure in the cylinder, that's 201 lbs holding the exhaust valve shut. The valvetrain must overcome that just to get the valve to crack off the seat. Then the spring seat pressure is added to that and multiplied across the rocker ratio (if such valvetrain configuration has one). Say spring seat pressure is 200 lbs, that's 400 lbs at the valve/rocker tip, and across a 1.5 rocker ratio would be 600 lbs at the pushrod and lifter. (on a center cam V engine) A 2.05" valve (common small block V8 intake size) has a surface square area of 3.3 in^2. Against 100 psi cylinder pressure, that's 330 lbs + 200 lb spring and then x 1.5 rocker = 795 lbs at the lifter. Even direct cam on tappet will see a tremendous difference. So having a smaller diameter exhaust valve is beneficial in reducing stress and shock on the valvetrain, thus also reducing torsional vibration, and increasing longevity/durability.

EDIT: To add a side bit of information... NHRA Top Fuel engines, making 11,000 HP with ~70 psi boost in a 500ci engine on nitromethane, are actually hindered significantly at that power level because of the exhaust valve and cylinder pressure. They have to open the exhaust valve way later than they'd ideally want to due to the extremely high cylinder pressure. They're opening it at ~75° BBDC when they'd ideally want to be closer to 90° BBDC. If they tried to open the exhaust valve at 90° BBDC, the cylinder pressure is still >1200 psi at that point. With a 1.925" exhaust valve with 2.91 in^2 surface area has 3,492 lbs holding it shut. Add 600 lbs spring seat pressure and multiply across a 1.53 rocker ratio, you have 6,260 lbs at the pushrod and lifter. Imagine jackhammering the pushrod, lifter, and cam lobe with the weight of a new F350, instantaneously, at 60+ times a second. You simply can't make a pushrod strong enough to withstand that. It would bend or snap just trying to open the valve. They have to wait until the piston is further down the bore, until cylinder pressure has lowered a bit more, before they can attempt to open the valve. When they have a catastrophic backfire, that lifts the intake and blower off the engine in a fireball, it's usually because something broke in the exhaust side of the valve train.
 
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Some very good answers.

A couple more. The intake valve is open less time. The exhaust valves often start opening before the power stroke is complete. The exhaust valve generally doesn't close until the very end of the exhaust stroke, and you don't want the intake valve opening until the exhaust is completely closed - or you get fuel on the wrong side of hot exhaust valve. So you have more time open on the exhaust side.

Also, smaller valves flow better relatively - so under the pressure of exhaust stroke the exhaust valve can move the same volume as a larger intake valve due to laminar flow under pressure.
 
I would give this one up to Bernoulli's Principle. There is an increase in dynamic pressure and therefore a decrease in static pressure is needed, lest the pressure in the exhaust would be higher than the intake.

By decreasing the exit area, flow velocity is increased, thereby pressure is decreased at a greater rate.

Volume and piston movement is the same, but the cylinder must be evacuated in the same amount of time to complete the stroke and start over.

In my limited knowledge on the subject, that is what I would in my answer box.
 
Thank you for the responses. We purposely chose a single-cylinder SI engine for simplicity and not a racing engine or a multi-cylinder engine.

In terms of Gas Dynamics (such as flow velocity, thermodynamics, piston crank angle, and pressures), explain why the Intake valve has a larger diameter than the Exhaust valve.

First, the piston works within a constant volume, which is the bore area X the stroke. Volume = Total Bore Area X Length of stroke. The volume above the piston crown is varying as the piston moves through a total crank angle of 720 degrees for a 4-cycle engine: intake, compression, ignition, and exhaust.

The intake valve is larger and the exhaust valve is smaller for several reasons.

Cooler air is denser, so in order to fill the cylinder as quickly and efficiently as possible, we need a larger intake area for the entering air. Since the piston is on its downward stroke, it is creating a vacuum in the sense that the dynamic cylinder pressure is falling below atmospheric pressure. Luckily, the viscosity of cooler air is lower than for hot air.

The exhaust valve area can be smaller since the combusted gas is hotter and more energetic, at a higher pressure, and will flow faster on the exhaust stroke. Thermodynamically, thermal energy wants to flow to a cooler area. In terms of gas dynamics, high-pressure gases want to escape to a lower pressure. Even though the hot gases have a higher viscosity than the cooler air, but being more energetic, they will move faster toward the exhaust headers.

For the valves, there is some flow restriction into and out of the cylinder and that is called the "curtain" area. The simplest expression for this is
Ac = pi X Dv X Liftlength, but in practice it is more complex because of the intake plenum and exhaust geometries. Dv is valve diameter.

In most cases, there is some degree of overlap between the closing of the exhaust valve and the opening of the intake valve. I.e., Some hot gas will flow into the cylinder on the intake stroke. This increases the volumetric efficiency of the engine since gas, like any fluid, has inertia.

The amount of lift and valve timing on each valve will depend on engine design goals. The amount of valve lift (opening length wrt the seat) is determined by the highest pont on the cam lobe x the rocker arm ratio.
 
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