Camshaft timing/tinkering (2001 Civic LX)

[dcb]

Originally Posted by garys_1k

If that was the case then we'd all tow boat anchors when accelerating just to increase loads as much as possible and get better FE.

There was a guy on GS who lives in the rockies and gets phenominal mpg (with a vtec ford stick shift). I have a few hilly spots on my route that I get terrific mileage through also so I suspect there is something to it.

The anchor analogy is interesting, but the engine stops having to work so hard once up to speed, unlike when climbing, so not exactly the same scenario, but might be an interesting test I have had some of my best runs with passengers in the car (EOC style of course).

[mechman600]

When you think about it, a hybrid car is sort of like a hill. The engine works harder than normal (because it's smaller than normal and is charging batteries) so it can turn off when running in EV mode when it uses no energy. Also, the newer multi-displacement engines are another example of "working harder = more efficient", having half the cylinders working twice as hard as normal during cruising speeds, saving fuel. A little bit, anyway.

[whokilledthejams]

Originally Posted by mechman600

I don't agree. The Prius has a smaller engine than most that works WAY harder than most, which makes the car efficient. The idea is that if you use a smaller engine that is working at its peak (moving the car, charging batteries, or whatever), it will be more efficient than a larger engine not working at its peak. An engine is most efficient at WOT at peak torque RPM, in terms of energy produced compared to fuel burned. Unfortunately, at WOT peak torque, any car currently produced will be accelerating vigorously producing way too much energy.
I just came back from a road trip where I drove the steepest, hilliest highway known to man, the Coquihalla (BC, Canada). Lots of time is spent going up 8-9% grades (to 2 summits) and coasting down the backside. Fuel economy always ends up being the best in this situation because the engine is working at peak torque uphill, and coasting down (using zero fuel). My normal 44 mpg (imp) went to over 50 mpg during this segment. Anyway, that's off topic. Back to my questions please.

James

The Prius has a variant of the engine in found in the Yaris/Echo/xA/xB1. However, instead of 105-108hp, it makes 76hp (IIRC). This is achieved through a valve cycle scheme that maximizes efficiency at the expense of power. As such, what mechman600 wants to achieve isn't completely insane. 50hp in a non-80s Civic would probably verge on undriveable, but changing the cams to keep the peak power at a low RPM would probably help a lot.

The Prius uses the Atkinson cycle, as opposed to the normal Otto cycle.
Atkinson cycle - Wikipedia, the free encyclopedia

In effect, it lowers the compression ratio by keeping the intake valve open longer than an Otto cycle engine.

[Rower4VT]

So does anyone have a SOHC Honda where retarding the cam increased FE? I advanced the cam on my SOHC Honda engine (Acura Vigor) and low end torque is definitely up, and FE seems to be up...but that could be a placebo effect.

[Duffman]

I dont think its a placebo Rower, retarding the cam reduces the dynamic compression ratio wihich is not good for efficiency.

[Rower4VT]

That was what I figured. The torque from 800-2000 rpm is noticeably improved also...I can lug along in 5th ~5mph lower than I could before. Shift earlier also....accelerating on the flat I can shift at 1500rpm.

[Andyman]

If you advance the camshaft you might be advancing the ignition timing at the same time because the distributor is usually connected to the camshaft. Advancing the ignition timing can make the engine more efficient and may have a greater effect than the change in valve timing. If the engine has a crankshaft position sensor then the ignition timing probably depends on the crankshaft instead of the camshaft. In this case it might not be a good idea to retard the camshaft because the lower cylinder pressure requires a more advanced spark for optimum efficiency and there is no adjustment to change it. Possibly you could increase intake air temperature instead to speed up combustion.

If the camshaft is set more retarded then the engine can tolerate a little more advanced ignition timing without knocking. This combination is likely to give the best fuel economy. If you retard the camshaft and it retards the ignition timing at the same time, I would expect lower fuel economy.

I have another Honda with a SOHC engine, a '94 Civic with a 1.5 liter 8 valve engine and 5 speed transmission. It has an old worn timing belt. I tightened the belt tensioner pulley so it wasn't as loose. That has a side effect of retarding the camshaft a little. Then I advanced the ignition timing a little. I took a long trip and the car averaged 50 MPG when I used about 10 gallons to go a little over 500 miles. I don't know what it would do with a new timing belt which would probably advance the valve timing a little.

I think that there are two reasons that retarded valve timing should help efficiency, at least at low RPM. One is that a later closing intake valve reduces vacuum during the intake stroke, reducing pumping power loss. The other is that a later opening exhaust valve increases the expansion ratio during the power stroke, recovering more energy from the burned gases before they are released to the exhaust system.

An ideal engine would have a completely vaporized fuel mixture, have no vacuum during the intake stroke (open throttle), compress the mixture as much as possible without preignition or detonation (about 8:1), burn all the fuel before the piston moves down, keep the heat in the gas instead of heating the engine, expand the hot gas until it reaches atmospheric pressure (which would probably require an expansion ratio of at least 16:1), and then release the exhaust at atmospheric pressure during the exhaust stroke (no exhaust restriction). The Atkinson cycle gets closer to this than a normal Otto Cycle. A retarded camshaft makes an engine more like an Atkinson cycle engine. Ideally the engine should have longer duration on the intake valve so the middle of the valve overlap period can be near TDC (piston at top dead center).

[Duffman]

Originally Posted by Andyman

If the camshaft is set more retarded then the engine can tolerate a little more advanced ignition timing without knocking. This combination is likely to give the best fuel economy. If you retard the camshaft and it retards the ignition timing at the same time, I would expect lower fuel economy.

The reason the engine is tollerating more ignition advance is that the dynamic compression is down. If the engine likes more advance but you cant accomplish this without lower compression then you need more octane.

Originally Posted by Andyman

I think that there are two reasons that retarded valve timing should help efficiency, at least at low RPM. One is that a later closing intake valve reduces vacuum during the intake stroke, reducing pumping power loss. The other is that a later opening exhaust valve increases the expansion ratio during the power stroke, recovering more energy from the burned gases before they are released to the exhaust system.

The vacuum that is at the throttle means diddly, its the vacuum at the top of the piston that is creating drag. Ideally the piston top sees atmospheric pressure for the whole intake stroke instead of near the end of the stroke. You need max valve lift early to minimize pumping loss throughout the whole stroke instead of just the end.

A late exhaust event makes for a lazy flow. This could be advantageous in that you may get more EGR which reduces your pumping losses.

[mechman600]

A very interesting topic...still. To better visualize it in my head, I paintbrushed a crude diagram. Hopefully the attachment at the bottom works for you.

From what I see, Andyman is right. In theory. Retarded = lower dynamic compression and lower pumping losses like the Atkinson cycle, and also a larger expansion ratio. Advanced = higher dynamic compression and higher pumping losses but lower expansion ratio. 1/2 dozen one, 6 the other. All theories, so I appreciate the stories of people actually testing the results. In reality, how big of a difference will it make if I retard/advance (depending on who's right) the cam timing? 5%? Who knows.

Does anyone know of a manufacturer who makes "economy" cams for a Honda D17A1? I really doubt it as all I can find is high performance ones.
Super low duration and little valve overlap would be good. Wait a minute...how about high-perf cams made for forced induction....they have very little valve overlap and less duration than typical high perf cams, but lots of lift. Would one of those work in an economy minded engine?

[edit] Nevermind. I just looked at some specs for forced induction race cams and they have waaaay more duration than stock ones. I should have known.

James

[Andyman]

Originally Posted by Duffman

The reason the engine is tollerating more ignition advance is that the dynamic compression is down. If the engine likes more advance but you cant accomplish this without lower compression then you need more octane.


The vacuum that is at the throttle means diddly, its the vacuum at the top of the piston that is creating drag. Ideally the piston top sees atmospheric pressure for the whole intake stroke instead of near the end of the stroke. You need max valve lift early to minimize pumping loss throughout the whole stroke instead of just the end.

A late exhaust event makes for a lazy flow. This could be advantageous in that you may get more EGR which reduces your pumping losses.

Ok, I agree about the vacuum on the piston but a late closing intake valve does lower the vacuum in the intake manifold and that should be about the same as the vacuum on the piston when the intake valve is open. The late valve closing causes some of the air to return to the manifold at the end of each intake event. That lowers the average air flow. To regain the lost air flow so you still make the same power, you will have to open the throttle more. That lowers the restriction and the vacuum. The reduced vacuum would be applied to the piston during the whole intake stroke.

I also agree about the lazy exhaust flow when the exhaust valve is opened late. That would increase the pressure during the exhaust stroke which is another factor in the pumping loss. There is an optimum opening point depending on engine speed. For low speed, the valve should open later and for high speed the valve should open earlier. When I'm driving I only rarely let the engine get over 3000 RPM so I prefer to have the timing efficient for lower speed operation. It would be best if all the valve events could be adjusted while the engine is running. It's not an easy or cheap thing to do.

I like those valve timing diagrams.