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sault18 t1_j6dc1vp wrote

Reply to comment by hikingsticks in Eli5 why aren't gas only vehicles far more fuel efficient than before by Live_Strongerrr

Those high efficiency numbers are in lab conditions at ideal operating parameters. A lot of these high efficiency numbers come from test engines that are years away from being in use or are so exotic that they will never be used in a production car.

In the real world, car engines hardly ever operate at their maximum efficiency point. Add in idling, engine warm up time and the inability of conventional gas cars to recapture energy from regenerative braking and the actual efficiency is much lower.

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hikingsticks t1_j6doxp3 wrote

Of course, yes. But those same factors affected older cars as well. Peak efficiency in lab engines is higher now than it was 20+ years ago, and efficiency in terms of real life use is also higher now than it was 20 years ago.

Hybrids start to bridge the gap with regenerative braking, but then of course its not purely an ICE vehicle anymore. Technically if its not a plug-in hybrid then all of the energy used came from the ICE, so you could argue that its an ICE vehicle with additional efficiency technology installed. That addition alone significantly improves the efficiency in terms of miles per gallon (or equivalent metric), but doesn't alter the thermal efficiency of the engine itself.

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sault18 t1_j6dscmt wrote

>but doesn't alter the thermal efficiency of the engine itself

Slight correction. Hybrids like the Prius use a slightly different engine cycle than pure ICE cars. Basically, the intake valve closes later in the Prius engine so the engine doesn't compress as much air as if it intake valve closed right after the intake stroke finishes. This means the expansion ratio is greater than the compression ratio, trading power for efficiency. The addition of 2 electric Motors helps make up for some of the loss in power. The electric Motors/ generators also allow the car to control the gas engine speed and not rely on the throttle so much to do so. This allows the car to run with the throttle more open and even wide open a lot more of the time. This reduces the pumping losses since the engine is not sucking air through a restricted throttle a lot of the time. To top things off, this also reduces pressure drop that would normally happen across the throttle in a conventional ICE car, increasing the air available in the cylinder.

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hikingsticks t1_j6dvtfd wrote

That's a good point about allowing the engine to run closer to peak efficiency more of the time, rather than having to do exactly what the driver requires. The buses in London are all hybrid diesels, and you hear them pull away from each stop on the electric motors, then the engine starts up shortly afterwards and trundles away as needed. So they can avoid the need for peak power on acceleration which tends to be more inefficient. It probably leads to increased reliability as well.

Regarding the valve timing, from memory that's called the Atkinson cycle. It's also used on non hybrid setups sometimes. I used to have a Peugeot 307 2.0 petrol engine that ran exactly like that, and I've encountered it in a lot of other engines as well over the years. As you said it's a reduction in maximum power output in order to increase efficiency.

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sault18 t1_j6dysos wrote

Yeah, the ability to accelerate from a stop up to 5 to 10mph on electric only really boosts city fuel economy. For a conventional car or bus, this is where they have the absolute worst fuel economy gulping down fuel in 1st or 2nd gear. I'd also suspect that bus drivers taking off from a stop in a regular diesel bus would step on the throttle and cause the engine to run a rich fuel / air mixture. So the benefits accrue here as well in the fuel efficiency and maintenance departments.

Going forward, I'm glad we're seeing explosive growth in all electric buses and cars. They're vastly more efficient and simpler than a hybrid drive train, avoiding the tradeoffs between power and efficiency hybrids had to make. Hybridization was a great technology for its time but it's increasingly being supplanted by full electric architecture.

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