Thackeray

Don't rev the engine to charge the hybrid battery...

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Here's an oddity I was unaware of.

Over the past few months of enforced idleness, it's been established in other threads that you don't need to drive the car to charge the 12v Battery. It's enough to set it to Ready state and leave it alone. You don't need to go for a drive unless you want to, as this will use more petrol than leaving the car stationary.

How to charge the hybrid Battery has also been discussed, though generally this isn't necessary unless you plan to park the car unused for several months. It's been suggested that if you want to do this, revving the enine might increase the rate of charge and intuitively it looked as if this must be right. But I was puzzled to find that when you do rev the engine the dashboard energy flow graphic shows that the charging stops when you increase the rpm of the engine. Then charging resumes when you release the accelerator to allow the engine to idle. I thought this might be to protect MG1 from excessive speed.

I've now discovered that this is wrong, courtesy of the data from Hybrid Assistant.

If the engine is not running, Hybrid Assistant shows around 0.2-0.3 kW are being drawn from the hybrid Battery (or 0.7 if you have the air conditioning compressor running). Just to try it out, I pressed the accelerator to check that you could rev the engine, even when it wasn't already running. Sure enough the engine revved up to around 1500 rpm and the Battery charging stopped, the same as if the engine had been running before touching the accelerator.

But then I noticed something strange. With the engine stopped the HA app was showing 0rpm and 0kW output for the engine and around 0.3kW drawn from the hybrid Battery to keep the car's electronics etc going. But when I revved the engine, I could hear the engine turning and these readings changed to 1500 rpm but still 0kW output. At the same time the Battery readings rose to around 1.5 kW being drawn from the Battery. This is power being taken from the Battery - not the amount of charge being fed into the Battery.

So what this indicates is that when you "rev" the engine, it isn't actually using any fuel. What's happening is that MG1, powered by the Battery, is spinning the engine and running down the Battery charge, rather than increasing it as common sense would suggest.

If this is all correct, the conclusion must be that to increase the charge of the hybrid Battery, it doesn't help to "rev" the engine. Counterintuitively, instead of charging the Battery, this will run the Battery down.

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How do you get the engine to rev when stationary? Mine won't do it, I think I've tried it on all positions on the gear selector. 

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You need to push the accelerator a bit harder than you might when driving normally. I also originally thought it wouldn't rev. But I found that if you give the accelerator a firm push, it will, on my car, at least.

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That should be easy to determine. Exhaust fumes, noise of the fuel injectors, ODBII reading of the injector cycle, O2 sensor reading etc.

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On 10/17/2020 at 6:06 PM, Thackeray said:

You need to push the accelerator a bit harder than you might when driving normally. I also originally thought it wouldn't rev. But I found that if you give the accelerator a firm push, it will, on my car, at least.

Mine will only go to just over 2000 rpm, but on reflection I don't need to rev it anyway. 

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36 minutes ago, Paul Brooksbank said:

Mine will only go to just over 2000 rpm, but on reflection I don't need to rev it anyway. 

I expect this must be a software limit imposed for various reasons. For example, if the engine is spinning at 2000 rpm, MG1 is spinning at nearly 8000 rpm. I think it can safely go a fair bit faster but I don't know what the limit is.

At the same time it's drawing energy from the Battery. Again this looks unlikely to be anywhere near a safe limit as it's only drawing 1.5 kW to turn the engine at 1500 rpm so it's unlikely to be much more than, say, double that amount.

In addition to that, all this torque in the planetary gearset means there must be a tendency for the car to start moving. The designers presumably didn't rely on the parking pawl in the gearbox to keep the car stationary when Park was selected and couldn't assume the parking brake would be on. So more counter balancing power must be fed to MG2 to keep the road wheels stationary. Otherwise, there's no reason for the car not to start moving when the engine is turning at 2000 rpm. I imagine these are some of the factors behind the limit of 2000 rpm.

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just for interest  (but off topic)  .... there is an 'inspection mode' you can put it in, and with that set different engine idle speeds depending on throttle position... used for diagnostics and emissions tests

For the Rx450h, the instructions are

Perform the following steps from (1) through (4) in 60 seconds.
- Turn the power switch on (IG).
- Fully depress the accelerator pedal twice with the shift lever in P.
- Fully depress the accelerator pedal twice with the shift lever in N.
- Fully depress the accelerator pedal twice with the shift lever in P.
- Check that "FWD MAINTENANCE MODE" is displayed on the multi-information display.
- Start the engine by turning the power switch on (READY) while depressing the brake pedal.

 

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