Are there 2 intakes or 2 exhausts?
Uh, Cylinder.
Intakes.
Better question, IMHO: why are these considered so special?
My 1992 Sentra and 240sx, my mom’s 1999 Frontier, and my 1998 Altima all have engines with 4 valves per cylinder. I thought by this time that most engines would. When I heard an ad extolling Ford’s new 3 per cylinder engines, I assumed I had to have misunderstood it. It doesn’t seem that way any longer. Why has it taken Ford and others almost 15 years to not even catch up with my 1992 economy four-banger?
I used to have a Corolla FX16, 4 cylinders with 4 valves each. That ratio made sense to me.
I would have guessed that the extra valve would be an intake, because there’s presumably less left after combustion to expell, but I’ve been wrong more times than I care to count.
Thus, my question.
Bambi H:
It’s not a matter of technology. Multiple valve engines have been around since long before 1992.
It’s a matter of priorities and trade-offs. Multiple valve engines cost more to build. They can deliver notably better performance and perhaps somewhat better mileage. So the question for the engineers and accountants is, do the benefits (more sales, better image [= more future sales], better corporate fleet economy) outweight the costs (R&D, tooling for new engine design, extra expense to build said engines).
Basically what’s so special about them now is Ford, having decided they need them for some reason (see above), is exuding marketing hype to make sure you know what an innovative, progressive, clever, and all-around wonderful company they are.
dnooman:
Intake port size is (virtually always) larger then exhaust port size because of the difference between flow rates under suction (intake) vs. under pressure (exhaust). I’m pretty sure the volume of gas involved is essentially equal.
To clarify re: port size – I’m referring to total per cylinder, irrespective of the number of valves used to achieve it.
At the 1912 ACF Grand Prix, Boillot won in his Peugeot L76 with OHC and four valves per cylinder.
Thanks, Gary. I knew they had been around much longer than that and figured that Ford was basically doing what you said. It strikes me like the changeover from carb to fuel injection. Not important until the Big 3 said it was.
Exhaust gasses are significantly hotter than intake gasses so the volume has to be greater. I am too lazy to research it right now, but I’d hazard a guess that exhaust gasses have 5-10 times bigger volume.
Well, in the cylinder, they don’t get to take up any more volume. Rigid walls and all that. They do end up at a lot higher pressure, though. It’s a good thing, too…otherwise the engine wouldn’t exactly work.
The higher post-combustion pressure allows the gases to escape through the exhaust valve easier than they are sucked in through the intake valve. The cylinder doesn’t create as high a pressure ratio (relative to the manifold) on intake. That’s why it benefits from more (or bigger) intake valves.
Another factor is flow “choking.” If you squeeze air through a constriction, it speeds up. (For initially subsonic flow, anyway.) If you squeeze enough air through a tight enough constriction, it will achieve sonic velocity at the narrowest portion–but never any faster. That’s called choking, and it defines the upper limit of the flow rate you’re able to squeeze through that hole.
In an engine case, we don’t want to choke the flow through the valves. Sonic flow through an orifice is generally something to be avoided. However, the total exhaust valve area is allowed to be smaller than the intake area, because the mugh higher temperature of the exhaust gas raises the local speed of sound significantly. Therefore, you can push the exhaust gases through without choking, at a velocity that would be impossible for the intake valves (they would choke first).
So, for any engine, you’ll always see more intake valve area than exhaust. This can be achieved by using bigger intake valves, or more of them.
There are 5-valve-per-cylinder engines out there, too. Like Audi/VW’s 1.8-liter turbo. That engine has three intake valves, and two exhaust.
Actually, 2 valves will flow more than 1 even if their total area is a bit smaller. The gases flow around the valve heads, not through them, so their circumference is more relevant than their area. The smaller valves can also be opened and closed faster at the same material stress level, since they’re lighter.
Honda Civics had 3-valve-per-cylinder engines in the 80’s.
Also, Ford’s been using 3-valve per cylinder engines at least since 2002! Mustang and Maurauder come to mind…
Let’s see… my own Ford has a 32-valve V8, so there’s 4 per cylinder. This isn’t new stuff at Ford!
Unfortunately, I’m out of the USA for now, and I don’t know what the commercial is touting… maybe truck engines? If so, here’s where the difference really is:
Pushrods. Pushrod engines have been around forever, and they’re bullet proof. All of the engineering and manufacturing bugs have been worked out of them years and years ago. When you look at Ford truck (and Crown Vic) reliability, you can know that a lot of the postive numbers are due to things that just don’t go wrong in these engines. Trucks always have the most reliable engines.
I wonder, then, if maybe Ford managed to build a pushrod multi-valve engine? I can’t envision a truck with overhead cams – not because it’s wrong, but just out of custom. It doesn’t “feel” right to think about it!
What you’re referring to is “curtain area”. Two small valves with lower opening heights can (potentially) flow more air than one large valve with a higher “lift”. And, as has been pointed out, the smaller, lighter valves have less resitance and the engine can be revved higher. At one point, Honda was producing 5-valve-per- cylinder motorcycle engines…3 intake, 2 exhaust, I think.
The reuced lift also allows higher compression ratios without creating an interference design, Peak acceleration in the valve train is reduced at any given rpm allowing for higher rpm and/or lower valve spring rates.
Yamaha’s Raptor ATV engine is a 5 valve design. 3 intake, two exhaust.
The record, AFAIK, is 8 valves per cylinder, on the Honda NR500 Racebike, circa 1979. Except they weren’t really cylinders. Is that weird?
Nearly all current Mercedes-Benz passenger car engines are 3-valve. I recall reading that they planned to return to a 4 valve layout over thenext few years, though. What is really strange with this is that in the late 80’s through the mid 90’s, everything was 4 valves per cylinder. The switch still has many Benz followers perplexed, as does dropping the phenomenal 3.2L inline six for a V6. Toyota is doing the same thing with the GS/IS/SC 300, too. Why?! I heard that it was customer perception: since every other car out there (except BMW, of course) has a V6, they must be better. :smack: