This happened with a warm engine, even though it was a cold day. I had been driving for more than 30 minutes in a mix of highway and main roads when I felt a slow, steady pulsing in the ICE. I had been approaching a red light when I allowed the car to slow down. The light turned green, and I was just starting to slowly accelerate. The battery was showing 7 green bars, and the ICE was not engaged (at least according to the MFD) as I gradually accelerated from the low-30s up to 40 mph. As I hit 40 mph, just before I would expect the ICE to kick in, the surging started. I was quite surprised. The surging stopped as soon as I increased the rate of acceleration. I've never felt the surging before (or since), and I'm wondering if anyone knows (or can guess at) the cause for this surging actvity.
Guess? I can play that game for hours I'm good at Speculate, too. My guess is going to be that you felt the engine starting up and taking the load. 41mph is the point at which the engine must be spinning, so if it isn't on already it will fire up. If you don't have a tach, the only way of knowing is that you will feel a "thump" or perhaps...maybe...a "surge" while the car makes the transition. The engine and wheels, and electric motor are always connected to one another via the PSD. I think that's why a particularly attentive driver feels engine start/stop events; it's not isolated like it would be if it were a regular transmission in neutral. Part of the reason I like having a tachometer is that it takes much of the mystery out of the car's behavior. If I feel the car do something and see the needle jump at the same time, I don't give it a second thought.
Just an Observation, Really I don't think I felt it just picking up the load. I really was just headed to 40mph, and I was on a level road. My acceleration was quite slow. I felt the gentle surging for quite some time. It felt to me as though it would continue unabated, and it only went away when I pressed to accelarate to a higher speed. I suppose I'm just posting an observation, yet I'd be curious to hear opinions as to why it happened. I had not experienced any surging before, and the circumstances didn't seem to match up with what I'd read in earlier posts. In most other posts, I'd seen a relation to cold engine, and that clearly didn't exist here. Also, I was at a very high SOC on the battery.
I think what he was trying to describe is a repetitive restart-shutdown, as though the load demand keeps oscillating just above the start-trigger and below the shutdown-trigger; I'd expect the car to be intelligent enough to require a cycle time or some other delay between restart events, but maybe his car is doing like my old microwave did every once in a while? ~ dan ~
Certainly, this could be the case, though I really do not know. I'm just commenting here on an interesting experience. I was trying to stay at or just below 40mph, though, and the MFD did not show that the ICE was engaged. Now I know the MFD doesn't show everything, but the surging was happening while I held right at 40mph on a level surface where no ICE involvement was showing on the MFD. I thought I felt the ICE, because I felt the surging and believed it was the ICE engaging.
One of my 'lessons learned' is to keep a guard-band around 42 mph. I'll slow down to 38 mph or speed up to 46 mph but I try to minimize transitions through 42 mph. My mileage data using cruise control to govern the speed showed that 42 mph in city driving gives a the worst mileage. Yet in controlled tests on level roads without other traffic, 42 mph fits on the drag curve. Bob Wilson
Internal Combustion Engine The Hybrid Synergy Drive switches back and forth between using the ICE and electric, and just the electric, as needed. The trick to really awesome MPG is to use the ICE as little as possible, and thus very little gas. The ICE automatically engages at 42 MPH, in order to be available (without shearing off a driveshaft or something like that) when needed by an increased load on the engine. [Disclaimer: I am not a mechanic, nor do I play one on TV. There are mechanics here that can explain the HSD much better than I]
In the hybrid world, it is an acronym for Internal Combustion Engine. In everyone elses world, it is frozen water.
The surging around 40mph is normal behavior. In fact, I have yet to drive a Prius that doesn't do it. I have a habit of driving on secondary roads right around the magical 40mph surging point on the Prius, where the powertrain starts constantly pulsing like you describe. I can duplicate it in less than five minutes. Don't sweat it. They all do it. It's normal. Try to keep your speed a bit higher or lower.
I felt it for the first or maybe second time today. It's the first time I can be certain I felt it. And I figured out and can replicate exactly the conditions to make it happen: accelerating somewhat hard up a hill (parallel hybrid at least), and as you top out at or around the low 40s, you let off the throttle to start decelerating. The ICE shuts down. But then you start demanding a bit more power, right as it's shutting down, and it will hesitate -- as if it can't decide whether to be starting or stopping. It was much easier in the earlier version of the car; in fact, that's essentially what caused the occasional startup lag when in ready mode with engine already shut down, in thinking about it. ~ dan ~
If you watch rpm as you coast down from 42-40, rpm briefly jumps from 960 to ~1180. There is some sort of fuel squirt around that threshold even if you're coasting. I think this happens if you're coasting down a hill and go from 40-42, can anyone verify? But what Bob said, minimize crossovers between 40 and 42 in either direction. I try not to cross 40 accelerating unless I know I'm going all the way to 50 or higher.
