ICE efficiency of Prius vs Insight

[bredekamp]

Hi all,

I was reading Hondas' website and find it interesting that Honda seems to have done more than Toyota to make the actual ICE more efficient. For example, does the Prius actually seal off the intake and exhaust valves of the engine during regen? This supposedly reduces pumping losses. What's also interesting is the use of two spark plugs per cylinder.

What do you think? Who's able of getting more energy out of the actual fuel? I still think Toyotas HSD is superior to the Honda IMA, but it looks like Hondas' ICEs are more advanced.

[Mike Dimmick]

Originally Posted by bredekamp

I was reading Hondas' website and find it interesting that Honda seems to have done more than Toyota to make the actual ICE more efficient. For example, does the Prius actually seal off the intake and exhaust valves of the engine during regen? This supposedly reduces pumping losses.

Honda's system has to spin the ICE's crankshaft to generate power, because the motor is bolted onto the end of the crankshaft. Therefore they have to close the valves to prevent pumping air.

Toyota's system is simpler: the crankshaft is completely stopped. MG2 can spin without spinning the engine. The car does spin the engine above 42mph to prevent overspinning MG1. The three components' spin rates - ICE, MG1 and MG2 - are related by the equation:

MG1 = 3.6 x ICE - 2.6 x MG2

See Understanding your Prius - The Power-Split Device.

The real big difference between the engines is how they approach saving fuel. Honda's is a direct-injection lean-burn engine, which means that it injects fuel directly into the combustion chamber, and injects less fuel than the stoichometric ratio of 14.7:1. The stoichometric ratio is the perfect burn, minimizing NOx, HC and CO. A rich mixture (excess fuel) burns incompletely, as there isn't enough oxygen, so CO is formed rather than CO2 and some fuel (HC) doesn't burn at all. A lean mixture (less fuel) burns hotter as each HC molecule is closer to an O2 molecule on average, and some free oxygen bonds with free nitrogen producing NOx.

The HCH has a lean-NOx catalyst to capture the NOx, split into nitrogen and oxygen, and store the oxygen on the catalyst; it periodically has to run richer to produce more HC and CO to burn off the stored oxygen. The two spark plugs are to help ignite the direct-injected stratified charge properly - lean mixtures are harder to ignite. (Stratified - richer in some areas, leaner in others; rich areas protect the valves and piston head by burning cooler.)

The Prius keeps to stoichometric ratio, and concentrates on using more of the energy from the burning air/fuel mixture. In the combustion stroke, the burning gas expands, pushing the piston down the cylinder. The piston needs to travel a long way. In a regular Otto-cycle engine, the compression and expansion strokes are the same length - the expansion ratio has to be compromised so that the compression ratio is low enough, so the fuel doesn't ignite itself prematurely or unevenly, or explode rather than burning evenly (called knocking). The compromise compression ratio means there is still a substantial amount of energy left at the end of the expansion stroke, which is wasted. The Prius Atkinson Cycle engine changes the valve timing so the intake valve closes when the piston is already part-way through its compression stroke, causing some of the air/fuel mix to be pushed back out of the intake. That allows the actual expansion stroke to be much larger without risk of knocking, and more of the expansion to be turned into crankshaft motion.

The downside to the Atkinson cycle is that peak power is reduced relative to an Otto cycle of the same capacity (because some of the intake air is always pushed back out - this makes the Atkinson really equivalent to an even smaller Otto engine). This doesn't matter in the Prius, as the motor can take some of the load.

The 1999 Prius reportedly produced up to 36.4% of input energy at the engine output shaft. (See attached graphic.)

Another major issue is the partial power problem. Conventional petrol engines have to use a throttle - a device that constricts the intake diameter - to control the amount of air (hence fuel) that enters the engine. This causes power loss as the engine has to suck air through a narrower gap, which requires more energy (try sucking air in through a straw - then putting your finger slightly over the end). At average speeds, conventional cars are throttled right back, causing appalling efficiency.

Direct fuel injection with lean burn can tolerate larger amounts of air in the cylinder, so the throttle doesn't need to be closed as far (in fact, diesels, which are also direct-injection, typically don't have a throttle - or it's referred to as a 'shut-down valve' - and the air intake is always wide open, typically turbo-charged for even more pressure). The Atkinson-cycle, since it pushes some of the air/fuel mixture back out, also doesn't need such a tightly-closed throttle for a given power output, but the 1999 result still shows a drop to 25% efficiency at 5kW output. (Again, see attached graphic.)

At top load/speed requirements, the challenge is to make best use of the oxygen that can be brought into the cylinder. Both engines make the fuel mix slightly over-rich to ensure that all the oxygen is used, at the cost of greater HC/CO emissions.

The 2010 Prius (and HS250h and RX450h) adds Exhaust Gas Recirculation. Exhaust gas is essentially inert - it doesn't burn, because all the oxygen in it has already been burned. Mixing in some exhaust gas with the air intake allows a greater quantity of gas to be brought in while injecting less fuel, but still burning at the stoichometric ratio for low emissions. It should allow an even wider-open throttle. It's been suggested to me that a small amount of exhaust gas injected at the right time blocks the flame's access to some of the oxygen in the incoming air, rather than actually diluting the whole intake stream. The pipe leading back from exhaust to intake doesn't look big enough to dilute the air intake much.

These are two different approaches, and neither one is bad. It might be interesting to see the result of coupling the Prius HSD layout to a lean-burn direct-injection engine.

[a priori]

Great response -- and a very good post! Thank you.

[bredekamp]

Thank you for that thorough explanation. I was also wondering what would happen if you connect a Honda engine (like the one in the Insight) to a HSD from Toyota.(nice hacking project for a brave soul)

It would seem that Honda swapped simplicity of the motor/generator for complexity elsewhere. Again, different approach similar result.

By the way that dual spark plug setup is used in the Alfa Romeo Twin-Spark as well. Wouldn't it be cool if we could just ditch the piston engine in hybrids, for a gas turbine in a series hybrid configuration

[FireEngineer]

I believe the current HCH also uses an Atkinson cycle to some extent, though not as much as the Prius. And lean burn is there in highway driving but not as aggressive as the old Insight manual which spewed NOx going at up to 18.5:1 lean burn.

Wayne

[qbee42]

Originally Posted by bredekamp

Thank you for that thorough explanation. I was also wondering what would happen if you connect a Honda engine (like the one in the Insight) to a HSD from Toyota.(nice hacking project for a brave soul)

It would seem that Honda swapped simplicity of the motor/generator for complexity elsewhere. Again, different approach similar result.

By the way that dual spark plug setup is used in the Alfa Romeo Twin-Spark as well. Wouldn't it be cool if we could just ditch the piston engine in hybrids, for a gas turbine in a series hybrid configuration

You would get worse mileage. The Prius engine is more efficient than the Honda when used with the HSD. Without the HSD the Prius engine would be pretty much undrivable.

Tom

[FireEngineer]

Originally Posted by qbee42

You would get worse mileage. The Prius engine is more efficient than the Honda when used with the HSD. Without the HSD the Prius engine would be pretty much undrivable.

Tom

I believe Hobbit had a Prius at around 40-45MPH steady and somehow took the MG's out of the equation. He reported about 10MPG less than with the full HSD. Agreed that from a start or accelerating without HSD this would be one VERY slow car.

Wayne

[Flying White Dutchman]

good post Mike Dimmick

[ken1784]

There is a comparison study between Prius and Honda IMA drive train.
http://yp.wtb.tue.nl/pdfs/4996.pdf

Some bsfc(g/kwh) numbers are shown at Fig-5.
Prius atkinson cycle engine is far better than Honda otto engine.

Ken@Japan

[patsparks]

The most important thing is, does it have centre vents to the back seats?