Steel188 said:
Please forgive the curiosity of an Englishman, but what exactly is the speed limit on your I5?
From the information available from Google over this side of the pond, it appears to be 65mph, at which speed you will get 36-40mpg (UK), which supports my long run figure of 32-34mpg at a SatNav regulated 72mph (when/where safe to do so).
Officially, it's 70 mph. In a practical sense, actually doing 70 will get you the stink eye from virtually every driver out there, as most people do at least 75-80. If you're cruising along in gaps in traffic, a lot of people will do 90+. It's basically a ~200 mile stretch from the just east of SF Bay Area to Grapevine, through the San Joaquin Valley, that's flat, straight, has no towns whatsoever, and everyone just wants to get to their destinations ASAP. It passes through only farmland and every 10-30 miles or so, there's an exit with fast food and gas stations. Then it's back to just passing through farmland. It think there's probably fewer than 20, perhaps maybe fewer than 15, turns in the road in that ~200 miles, and most of them are very gentle turns of no more than 10-15 degrees over a distance of 1000 feet or more as the road realigns. And you can even do the "turns", if you want to call them that, at 90+ mph easily. And really, after doing more than 50 miles of conditions
like this and knowing you have 150 more to go, you will want to.
And since Tai626 has driven the Cajon Pass on I-15, I'm sure he knows about
this spot which is just as bad as I-5. The horizon literally does not change for 10+ miles and it makes you just want a car with a higher top speed so you can get it over with.
Steel188 said:
I don’t profess to know the formulae by heart, but drag increases exponentially with speed, and our cars are about as aerodynamic as an 1800Kg house brick, so is it any wonder your fuel consumption is so bad?
A good way to think about this is you learned about kinetic energy in physics, right? KE (non relativistic) = 1/2*m*v^2. When you are pushing through the atmosphere, each air molecule hits you with that amount of kinetic energy and slows you down. Since the force at which the air molecules are hitting you is proportional to v^2, drag is quadratic with respect to speed. Power (amount of work you need to do to travel a certain distance) however is force dotted into the displacement, in other words, F*Δx. F is proportional to v^2, and Δx is proportional to v, so power is proportional to v^2*v or v^3. Thus, to go twice the speed requires eight times the power. In more layman's terms, you can think of it like this: when you double the speed, each air molecule hits you four times as hard (since their kinetic energy is proportional to v^2) AND you're pushing through twice as many of them per unit time as well, since you're covering twice the distance. 4 times the kinetic energy * 2 times the number of air molecules hitting you = 8 times the power
As far as why fuel consumption is so bad, well, it's just physics. A lot of it, at least. But, you don't have to make it worse than physics requires it to be. All engines have a
consumption map that shows how much fuel is consumed to produce each unit of power output. This is called
brake specific fuel consumption. On the X-axis is engine RPM and on the Y-axis is engine load. You can see that there's a "sweet spot" where the engine has the best BSFC and it is typically at a high load and fairly low to medium RPM.
Here's a typical consumption map:
View attachment BSFC.png
Red indicates lower fuel consumption; orange, yellow, green, blue, and violet and black are increasingly bad fuel consumption (more fuel consumed to make the same amount of power). Now notice several things about this engine:
1. If you want the engine to make 124 HP of power, you have only one choice: it MUST operate at between 5500-6000 RPM and at a fairly high load.
2. If you want the engine to make say 40 HP, there are a lot of ways to get it (the blue curved lines are the lines of constant power output, and we can be anywhere along the 40 HP line). You can operate it at a low RPM and high load, or at a high RPM and low load. Putting it at 2200 RPM at a fairly high load makes it extremely efficient. But you can also operate it at 5000 RPM and under a fairly low load and it will eat a ton of fuel (this is the region we wish to avoid). This is because a lightly loaded engine at a high RPM not only has to overcome more internal friction, but must pull a vacuum against a mostly closed throttle.
So now you can probably see why I'm annoyed with the fixed ratio clutch. The clutch is designed to allow the engine to make peak power at around 90 mph, and at this speed, it turns at around 3750 RPM. But if you aren't climbing hills, and you are cruising along at between 78-106 mph, you don't need all that power. Normal transmissions would shift to a higher gear to lower the RPM and put the engine back into the red or orange region, but the PHEV cannot do this, so it's forced to operate the engine in the yellow or green region (by design, it cannot use series mode, and lower the engine RPM, if it is traveling at or above 78 mph). It *tries* to be slightly more efficient and since it cannot reduce RPM while keeping power constant, it increases load while keeping RPM constant; in other words, it tries to move up on the *power* curves at the same RPM. It runs the generator and dumps excess power into the battery (this puts the battery through useless cycling too, but let's ignore that for now). But once the battery fills up, it has nowhere else to put the power, and it just runs inefficiently.
