CVD Efficiency
Some things are hard to explain…
Written by Steve Arnold • 5 min read
I’ve been working on the CVD engine on and off since December of 2008 and in these past 11+ years, I have had dozens of conversations with some of the brightest engine engineers in the business about the most important aspect of the CVD engine. I have generally failed at explaining this aspect well enough to get the light bulb to go on for most of these colleagues. At least the light bulb that I want to go on. Any rational person would find this troubling and I’m sometimes rational, so it bugs me. But I’m not about to give up. Progress is being made as one can and should improve after 11 years of practice, thus I’m getting better at explaining this crucial aspect of the CVD. At least these discussions don’t get heated any more as I recognize that when the other person’s heels start making black marks on the floor, I’ve lost another skirmish and I should move on.
Part of the problem is what we all were taught in school—the “thermodynamic cycles”, especially the Air-Standard Otto Cycle. If you don’t know about the Otto Cycle, this blog is probably not going to be very interesting unless you go get a copy of Van Wylen and Sonntag’s Thermo book and brush up on Chapter 9. Even then you might want to take a pass…
Anyway, the Otto Cycle is a thermodynamic cycle which purports to represent the SI engine. But it is a closed cycle and thus has no air coming in or exhaust gas going out or fuel going in and combusting. And since it has no inlet or outlet, it does not have inlet and outlet pressures and temperatures. They are “state” points which don’t provide for differences that exist with a real engine with real pressures and temperatures that are different between inlet and outlet manifolds. There is also no control of the cycle and it has isentropic processes which happen at an infinitesimally slow rate. Other than these trifling points, it’s perfect! A friend and colleague pointed out that on Wikipedia, the Otto cycle is described not as a closed cycle, but with the intake and exhaust processes. I see NASA has a similar write-up. Probably written as disinformation for the Russians. I assume the Wikipedia article was written by someone trying to reconcile the true Air-Standard Otto cycle and the real mechanical cycle of an SI engine. It makes perfect sense but it doesn’t help me tell the story I want, so I’m rejecting it. I’m sticking to Van Wylen and Sonntag, which I assume you have a copy of since it seems every engineer’s office I walk into, it’s sitting on a shelf like that Gnome that shows up everywhere. When I see it, it always reminds me of all the thermo I either didn’t learn or forgot after the final. You don’t have to remember much these days since you can Google just about anything. Google doesn’t provide the understanding though. But that’s off-topic—I need to get back to my story.
Anyway, assuming that we have swallowed the Air-Standard Otto Cycle as a representation of the SI engine, we arrive at the Holy Grail of equations for engines, that is, the efficiency of the engine is a function of the compression ratio.
Alas, that purportment is not exactly right either. Any engineer worth his pocket protector will tell you the efficiency is a function of the Expansion Ratio, not the Compression Ratio. But since the pressure ratio and the expansion ratio are equal to each other on a Volume basis (State points 2 and 3 have the same volume, as do State points 1 and 4, therefore the compression ratio 1—>2 and expansion ratio 3—>4 are equal), we can let this little faux pas go. However, if we look at it on a Pressure basis all bets are off and this doesn’t hold true. You can scroll back up and deduce for yourself from the diagram. And for me, Pressure is what it’s all about. Expand from high pressure to low pressure. Of course, I’m a turbo guy and that is how a turbine works, so I’m sticking with pressure!
So, this whole purportment (not really a word, but you know what I mean) about the Otto Cycle and the SI Engine Cycle depends on looking at the processes based on a Volume basis. For the Otto Cycle, this means that we start with the piston at bottom dead center, and we rotate the crank 180 degrees, and the ratio of these two volumes is the Compression Ratio (and Expansion Ratio). That’s the fundamental premise of this teaching. Really. It has absolutely no resemblance to reality. Again, a closed volume at BDC being compressed to the closed final volume at TDC. Nothing remotely close to this happens.
As everyone knows, the valves don’t open and close instantly at TDC and BDC with the mass transferring at warp 9. The valves are opened and closed by the cam. In fact, engines never have the air compressed starting at a maximum closed volume and finishing at a closed minimum volume, with the theoretical compression ratio so noted in the Engine Specifications and proclaimed proudly in the engine specs. The volume-based compression ratio is really meaningless in an absolute sense. Everyone knows that, right? As least indirectly if they understand cam timing.
To make things worse, engineers can’t keep from diddle-farting around and make the cams open and close at all sorts of odd times. These VVT mechanisms are incredible—but they can only make the closed volume (displacement) smaller, not larger. Stop for a moment to consider this—variable valve actuation can only reduce the volumetric efficiency, and can only reduce the compression pressure. Is that what we’re looking for? I thought we wanted a high compression ratio so we would have a high expansion ratio, and thus high efficiency, right? Please don’t throw your coffee mug and damage your computer, or yell at me and and scare your spouse. We’ll get back on-track soon and let your blood pressure return to normal.
So, what does the X-axis actually mean with all of this engineering subversion? There is no closed volume at the supposed displacement of the engine—ever. The compression ratio that we’re so proud to specify doesn’t exist within the engine.
The X-axis has lost its identity and has no idea who he/she is. Surely, X must represent something! How about the piston motion? That could be OK, except that is a linear quantity and in the PV diagram, it is a cubic quantity. Since this whole Otto Cycle thing has played fast and loose with the rules, we could overlook that indiscretion.
Now maybe you see my problem. I’m trying to develop a piston drive system that changes both the maximum volume and the minimum volume of the engine. It’s wonderful—it really is. Except I just explained how those volumes don’t exist. Maybe that’s why other engineers struggle to understand what I try to explain. It’s not you, it’s me.
Let’s try a different approach. Intuitively, we know that a high compression ratio results in a high compression pressure as long as the starting pressure is near atmospheric. With a given amount of combustion energy, high compression pressure results in a high starting pressure for expansion. If our exhaust manifold is connected to something close to atmospheric, then that will result in a high expansion ratio, thus high indicated efficiency. Conversely, if we throttle the intake so that our intake manifold is at 1/2 atmosphere, we will have half of the compression pressure. With the same amount of combustion energy (fuel), this will result in a much lower starting pressure for expansion. Our ending pressure, near atmospheric pressure at the exhaust did not get reduced in half like our intake manifold pressure, so we will have a significantly lower expansion ratio—lower start, higher finish. And thus, much lower indicated efficiency. This is the fundamental issue with SI engines—controlling the engine by reducing the intake manifold density has a direct and substantial negative effect on indicated efficiency.
It’s time to pause and reflect. The effect we just walked through showed a significant reduction in indicated efficiency from throttling the intake manifold. And we never once mentioned pumping work. Of course, pumping work is the first thing an engine guy thinks of when they are asked what loss the throttle is responsible for. But, pumping work is a distant #2 loss. Engine Simulation shows the #1 loss created from the throttle is exhaust waste heat. And a lot of it.
When we look at the GT-Power simulation of the CVD and a Baseline V8 done by Brad Tillock and EngSim, and focus on Mode Point 1, we see an 80% improvement in Indicated Efficiency. And from the Fuel Weighted FTP Chart, we see an improvement in friction (ball bearings, rods, piston side thrust, piston speed). The rest of the improvement is from the elimination of throttling, which 1) is the pumping power that everyone thinks of, and 2) a huge reduction in exhaust waste heat due to the higher compression pressure at part load (which translates to high expansion ratio). An important fact to note here: this improvement is due to increasing the compression pressure by eliminating throttling and downsizing the engine so it runs at full load. This gain cannot be achieved with any type of variable valve actuation or Valvetronic system as they can only reduce the pressure, not increase it. I’m ignoring acoustic wave effects which are useful at full load.
To be honest, I don’t really care if my little Otto Cycle rant is true or not. All I really care about is that you agree that use of a throttle is the root cause of poor SI engine efficiency over an ftp cycle and it can’t really be fixed without variable displacement. A poor man’s variable displacement is cylinder deactivation. It doesn’t reduce the piston speed, or have a big reduction in friction like the CVD, and the challenges of running on one cylinder—well, that’s a topic for another blog.
As a turbo engineer, it pains me to say this, but the boosted, downsized, down-speed, dilute SI engine is a very expensive and roundabout way of eking out some fuel economy. There’s another blog topic. That is, assuming I don’t get skewered for this one and have to go into hiding.
Efficiency is our forte.