Gas Engine Starter Motor

Hi, I was wondering what starts the gas engine. I never hear anything like on a regular engine. Just curious to know how it works. Thanks.

The Prius has two motor/generators, called MG1 and MG2. When you first power up the car, MG1 starts turning and begins spinning the engine. When engine RPM and oil pressure reach acceptable levels, the engine lights, generally several seconds after startup.

One of the best places to read about how the car works is on the site of one of PC's members, john1701a.

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It's done by the generator, the engine opens its valves, so there's no compression/resistance, than the generator spins the engine to about 1000 rpm, valves are closed, then ignition. (hope I have it all correct)

Close - the Prius engine isn't sophisticated enough to totally open its valves; it can only change the timing of the intake valve (and not its open period), so the best it can do is adjust its timing to minimise compression and resistance as much as it can.

Well, I'd only add that "isn't sophisticated enough" really isn't a fair characterization, especially in light of all the other tricks this car plays (especially using it's intake valves radical timing adjustment range to, in effect, vary the engine's displacement -- a/k/a Atkinson cycle). Additionally car maker research has indicated that the most efficient way to "cut" cylinders temporarily generally involves leaving the valves closed. The trapped air that is compressed acts as a spring, returning almost all of the compression energy on the piston's downstroke, making the "dead" compression and "power" strokes pretty much an energy wash. By contrast, if the valves were to be "popped" open while the cylinder was unused, the engine would still be expending energy moving air through -- air that unlike when used in combustion, is now contributing nothing but more drag.

As a side note, those monstrously noisy "Jake Brakes" on the big rigs take advantage of this principle. When you hear that incredible "bwaaaaa" sound from a Jake Braked truck, what's happening is the system allows the dead cylinders (in this case all of them) to compress an intake charge (big energy, remember diesels have huge compression ratios compared to gas engines). Then, when the engine has spent all that energy compressing the air, the Jake Brake opens the exhaust valve, allowing the compressed air to explode from the cylinder (hence the awful noise), and not allowing the "springback" effect that would neutralize the net energy. This gives heavy trucks tremendous extra braking power, without the risk of overheating friction brake components.

But back on point, yes, the Prius uses the smaller of its two Motor-Generators (MG1) as the starter for the ICE. The 1NZ-FXE therefore has no "starter motor" in the sense that a traditional gas or diesel car does.

Certainly MG1 provides the ICE starting torque but not as well known, MG2 also provides a little bit of 'counter torque' to hold the ring gear stationary. This minimizes the stress on the parking paw. It is a minor detail.

Bob Wilson

If I understand correctly, the gas engine is started by the generator. And I imagine that the generator is powered by batteries (550 volts) and not the 12 volt battery?

correct.

The 2Gen and later HV battery is 201V I believe.

The battery voltage is nominally 201.6 volts, made up by 168x 1.2 volt cells. This voltage varies though as the battery state of charge does. I do not know the minimum and maximum figures but it could rise to around 240volts.

John (Britprius)

MG1 is powered by the inverter. The inverter is powered by the high voltage traction battery.

While converting to DC to AC, the inverter also increases the voltage from 201VDC nominal to three-phase ~500VAC.

200-244 volts, with a brief drop to 190 when the engine starts.

As usual Patrick, your post here puts a finer point to the issue, while I am still only able to paint the picture of most Prius issues with a broader brush. Your more detailed information is greatly appreciated and very helpful in coming to a better understanding of our fine cars. Thanks.

If you think of batteries only in terms of voltage then they're going to do mysterious things like this... and you'll be left wondering why and what and how.

But if you learn about Thevenin's theorem, then it will all become crystal clear, and one can calculate these minimums and maximums and even determine SOC with simple arithmetic.

Thévenin's theorem - Wikipedia, the free encyclopedia

Think of the battery having an internal resistor, whose resistance changes with SOC. All of a sudden, the fact that the voltage drops when the ICE is started... makes sense!

I'll leave the rest of the lesson to you as homework.

No reason to do any calculations, I posted the numbers I've actually measured 3 posts above (see my sig link for more info).

Sorry if I'm not being clear, but my point was thus: there's no reason to measure several voltages if you have the internal resistance as a function of SOC.

If all you think about is voltage, then the battery will be a mystery box that spits out various numbers at various times. If you follow Thevenin's theorem, you'll be able to anticipate (to first order) what the battery will do under various conditions. Hence, a Thevenin equivalent model is much more valuable than a few measurements.

Not for most people, sorry if I wasn't clear about that. For most people they just need to know the voltages which can very easily be measured.

Interesting!!! Is this why sometimes when my ICE starts, the car seems to want to lunge forward?

Yes MG1 in trying to turn the engine some of the energy tries to go through the power split device to turn the wheels and MG2 but the parking pawl stops this.

John (Britprius)

I guess this is why when half my inverter fried, the car would move forward slowly on electric but the engine wouldn't fire up, nor was regen braking working.