The Atkinson cycle engine was developed over a century ago. The aim was to maximize the power stroke of the cycle to extract more work from combustion. The original design used a complex series of linkages to modify the path of the piston so that the compression stroke was relatively shorter compared to the power stroke.
Modern applications of this engine look very conventional however. No linkages or additional complexity. Instead, they modify the timing of the intake valve. The point is to open the intake valve as the piston starts to move down, drawing in fresh air. Conventional engines close the valve as the piston reaches the bottom of the stroke, then compress all this air for later combustion. The Atkinson modification found in many engines such as the one in the Toyota Prius, closes the intake valve late. Now, the air enters the cylinder before a portion is pumped back out, effectively lessening the intake volume and increasing the power:intake ratio to achieve the Atkinson effect.
Now, I think this is backwards. The same effect, a smaller intake charge, can be achieved by closing the intake valve much earlier. A conventional Otto cycle engine closes the intake valve just after BDC, around 50 degrees ABDC or so. How about 50 degrees or more before? What effect would this have? Well, the intake valve closes while the piston is still descending. The piston continues to descend and now pulls a partial vacuum in the cylinder... The crank keeps rotating, the piston rises and eliminates the partial vacuum before continuing to create compression and the rest of the cycle.
Now, given the perhaps unmatched maturity of the spark- ignited gasoline engine, this is definitely a dreadful idea. I can't, however, figure out why. I see several advantages:
1. The early intake valve closing achieves the reduced- charge Atkinson effect, WITHOUT the pumping losses associated with intake and partial exhaust (de-intake?) through the intake valve. Why move the air in, if you're only gonna move it right back out again?
2. As the piston approaches the bottom of its stroke, it has to be decelerated, then it hangs around doing very little before it has to be accelerated back up the cylinder. By closing the intake valve early, the partial vacuum* applies a force which decelerates the piston then accelerates it again in the opposite direction. It acts as a spring.
You can demonstrate this with one of the large numbers of syringes that I'm sure everyone has on their work desk... pull out the plunger a little, block off the end... keep pulling and let go.. The atmosphere pushes it back. Newcomen wasn't a fool. So instead of wasting crank- derived energy pushing some of the intake charge back out, you save energy by moving less air.
While piston deceleration force is delivered to the crank, I reckon about this much:
Piston+wrist pin+1/2 rod = 0.64 kg Bore&Stroke 85 mm x 90 mm Force = 0.64x15,000ms/s = 9600 N (for 5000 rpm) Work Done 432 J/rev.
Vacuum/pressure gradient Piston area 0.0054 m2 Pulling 20% max vacuum, average 10% = 10133 N/m2 Mean Pressure force = 55 N Work Done = 2.5 J/rev
So about 0.5 % of the piston acceleration forces never move through the piston assembly. Hmm, nice but not a huge change.
3. The low pressure component of the cycle could increase fuel vaporization. Gasoline already boils close to engine operating temps, lower the pressure for a little while and this should speed up vaporization.
4. The intake cam duration is shorter. The intake valve spring is compressed for less time and the cam is consequently is doing less work.
Like I say, this is almost certainly a bad idea, but I don't know why.
*Or pressure differential between the crankcase and cylinder... if you're gonna be picky about considering the whole system.-- bs0u0155, Jun 01 2016 Early closing of intake valve, reduced pumping losses http://www.scienced...i/S0360544214012158 [bs0u0155, Jun 02 2016] Variable Valve Timing https://en.wikipedi...riable_valve_timing [bs0u0155, Jun 02 2016] Budack Cycle https://www.youtube...watch?v=JjM14JBPdBI [bs0u0155, Mar 03 2021] um... this might be a stupid question I'm not sure, don't you need the oxygen to make the gasoline ignite? By firing after creating a partial vacuum you'd be asphyxiating the gas... wouldn't you?-- 2 fries shy of a happy meal, Jun 01 2016 //don't you need the oxygen to make the gasoline ignite?//
Absolutely you do. I'm not pulling much of a vacuum, 80% of a normal intake charge is retained. The key to an Atkinson cycle is that the gasses from the burned fuel and air are much more than the gasses that went in. Normally, the extra gas in the same space creates excess pressure.... but once the piston gets to the bottom of the stroke, the pressure can't be translated into useful engine power, instead, it just waits until the exhaust valve opens and whooshes out with real vigor as waste heat, pressure and sound. That waste heat and pressure may be used to power the turbine of a turbocharger in other engines. The Atkinson cycle reduces the waste by restricting the air (and fuel, since the engine is designed to maintain a constant ratio of air:fuel for optimal combustion) that goes into the cylinder. This means that the smaller air&fuel combustion can expand more completely. This engine has a cooler, quieter exhaust, uses less fuel per unit of output power, and allows you to bore people in pub conversations. The principle downside is that the engine is totally gutless at low revs. That "waste" pressure of the standard, Otto cycle engine, makes it tourqy and punchy. Toyota didn't care too much with the Prius, since the electric motor does a lot of the low speed work and, how can I say this... the Prius customers had pre-managed expectations when it came to "punch".
A few manufacturers are now selling non-hybrid Atkinson cycle engines, which are using fairly fancy variable valve timing to switch the engine in and out of Atkinson mode depending on load.-- bs0u0155, Jun 01 2016 I suppose the best way to understand it is to think of a full charge, un throttled which is what we are working on from a thermodynamic perspective. Your design would achieve the same effect as a throttle does, reducing charge by restriction: the charge is still compressed at the same ratio as it is discharged. Instead you must reduce the efficiency of the compressor on the intake stroke (and increase the compression) WITHOUT forcing the engine to suck a vacuum which is where the benefit is gained.-- WcW, Jun 02 2016 //Your design would achieve the same effect as a throttle does, reducing charge by restriction://
I was thinking about this. A long time ago, while observing the long tortuous route the intake for my crappy-but- loveable 1998 Nissan Micra took on it's pilgrimage around the empty engine bay, I wondered if restriction was essentially designed in. One snazzy cone filter later and I had a noisier car with slightly worse fuel economy. So, possibly.
You are entirely correct about the throttle of course. What I've invented is an engine running at part throttle. The throttle in this case is the intake valve rather than the butterfly valve in the throttle body. Now, if you could vary the valve timing well enough, you could gain some control over the process... What I've invented there is variable valve timing for the intake, like BMW's VANOS. And that's already been invented.
If you had REALLY good control of inlet valve timing and lift, you wouldn't need the throttle body at all. Unfortunately BMW already invented that too, it's called Valvetronic. There are others too.
//Instead you must reduce the efficiency of the compressor on the intake stroke (and increase the compression) WITHOUT forcing the engine to suck a vacuum which is where the benefit is gained.//
Well, no. I dispute that pulling a vacuum is energetically worse than pumping air in then out again. Closing the inlet valve early does work quite well at reducing pumping losses. See intro of <link>. So, why is it a dreadful idea? Well, closing the inlet super early gives you a really short duration. That means you either have to have a really aggressive and inefficient cam profile, OR you go with much lower lift, which makes the engine poor at high rpm. The Atkinson cycle allows a lazy long duration cam profile and you actually need less lift for the same total airflow.
Cool, I learned something.-- bs0u0155, Jun 02 2016 So, at the end of the day, is there any difference between what a throttled engine achieves vs. an Atkinson ?
(but yes, this idea is pretty much obvious - if you want to restrict the aircharge in an Atkinson, why not do it on the intake stroke, to avoid pumping air around needlessly [edit: hmm, okay the intake valves don't have to work as frantically])-- FlyingToaster, Jun 02 2016 One advantage of your idea compared to throttling is that with throttling, the piston is pulling a partial vacuum for the entire stroke. Energy is lost as heat as air is forced through the throttle. With your idea, there is less resistance for most of the stroke. When the valve closes, it closes all the way, so air isn't being forced through a restricted opening, and the partial vacuum that develops is actually slightly useful in slowing down the piston.
The valve issues do sound like one possible reason not to do this though.-- scad mientist, Jun 02 2016 In the "Variable Valve Timing" link there is a section on "Early intake valve closing (EIVC)". It sounds like it describes this idea exactly. It reduces pumping losses by 40% and increases fuel economy by 7%. The downside they mention is that it increases hydrocarbon emissions because it reduced the cylinder temperature.
That seems strange because I always had the impression that other design trade-offs where made to prevent the cylinder from getting too hot, so something that reduces the temperature seems like a good thing.-- scad mientist, Jun 02 2016 so has anyone thought about a slurry of industrial diamonds and gallium lining a plenum at they ylinder walls to be ultra warmth conductive to reduce emissions? 1 part per hundred is like an could have fiscal value-- beanangel, Jun 04 2016 I always thought this was a jolly good idea of mine. It turns out VW have termed it the "Budack" cycle <link>. They've got around the problem of the lumpy cam using those fancy hydraulic cam-follower fulcrums.-- bs0u0155, Mar 03 2021 I recently saw (umm... something?) about the Budack cycle. A quick Google brings up articles from as far back as 2017, so you still beat them. Damn professionals, reading the halfbakery again...-- neutrinos_shadow, Mar 03 2021 //Damn professionals, reading the halfbakery again...//
I could be quickly silenced should a shiny new/well restored old VW camper were to be offered...-- bs0u0155, Mar 03 2021 Pulling a partial vacuum in the engine I would think would be bad for the oiling system somehow. I suspect you'd get a lot of cylinder wall um... what's the word... (tarnishing?) with this arrangement. I could be wrong, though.-- RayfordSteele, Mar 04 2021 random, halfbakery