ICE efficiency of Prius vs Insight

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.

 

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

 

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

 

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

 

good post Mike Dimmick

 

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

 

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

 

I would be interested in the reference.

I take a much simpler approach to taking out the electrical 'overhead' of the HSD:

• 95% efficiency - either MG1 or MG2 working as a generator
• 95% efficiency - either MG2 or MG1 working as a motor
• ~90% efficiency (.95*.95) - MG1 to MG2 or MG2 to MG1 efficiency
• ~10% loss (1-.90) - how much electrical energy loss occurs
• 28% power - the power split device take of the ICE power
• ~2.5% loss (.28 * .10) - total electrical loss by the HDS

All other losses are ordinary mechanical such as using the 'silent chain' or the oil pool lubrication. Both of these have been addressed in the 2010 Prius giving a 20% reduction in transaxle drag. There was not much to start with but this easily brings the HSD into regions where only the most optimized transaxles may compete.

The only thing not yet done in the HSD is a highway speed, one gear ratio, lock-up. This would eliminate the remaining 2.5% electrical loss and provide a little more than a 5% increase in mileage (0.025/0.40). At this point it is pure, ICE-to-ICE tweaking and there is still a lot of waste heat to exploit.

Bob Wilson

 

The underlying message here: engine efficiency alone doesn't count for much. What matters is the system efficiency (which of course includes the driver and the road).

 

I have just learned so much in these two pages. Thank you VERY much!

 

The main thing to learn is to never, ever ask one of us a technical question.

Tom

 

Another good read, courtesy of your tax dollars:
http://www.transportation.anl.gov/pdfs/HV/454.pdf

With an inline torque sensor at the flywheel of a 2004 Prius, they found the steady state thermodynamic efficiency at cruising speed (50-60mph) to be ~37%. My understanding is that is exceptional for a gasoline engine, and rivals many diesels. When comparing different engines its key to differentiate peak efficiency from steady state efficiency. Many engines achieve peak efficiency only under heavy load conditions, such as acceleration. The Prius engine appears to me quite impressive in its ability to achieve not only a very high peak efficiency, but to maintain a very high efficiency under lighter load conditions such as constant speed cruising where its of more practical value. This is a big part of why the Prius is able to still get quite exceptional highway mileage, even though the hybrid system is of little use there. The hybrid system is still the key enabler that allows the engine to be designed in this way, and so is indirectly responsible for the highway performance.

For comparison, here is a study that puts the efficiency of a VW 1.9L TDI in the range of 30-38% under low load, low rpm conditions:
http://www.ohiolink.edu/etd/send-pdf.cgi/Van Horn Charles.pdf?acc_num=akron1154354134

I've yet to find a good source, but my understanding is that although peak efficiency may be higher, a typical gas ICE cruises with efficiency somewhere in the 20s. I would expect Honda's lean burn engines to do better than that, but I haven't found a reference for that either.

Rob

 

Thank for that Pat, LOL!

 

Folks:

The latest gen hybrids from Honda do NOT use lean burn at all. Lean burn was only used in manual transmission equipped first gen HCH and Insights.

Cheers;

MSantos

 

The thing I don't quite understand in Honda's engine is why shutting both valves will reduce pumping loss? Isn't shutting both Intake and Exhaust valves in the compression stroke, thus higher resistance for the piston to go up?

 

Thank you!

I very much enjoyed this paper: learned something; figured out something else, and; a few things may need a little rework

#1 - EM1 torque limit

"The maximum torque of EM1 reduces the limitless range of torque-speed combinations transmitted through the epicyclic gear." (pp. 2/12)

Yes but the ratio remains fixed that roughly 28% of all power has to flow to or from EM1. This is fixed by the gear ratio of the epicyclic gear (aka., power split device.)

#2 - Rolling resistance 0.0054 (IMA), 0.009 (HSD)

Table 2: Parameter values for the simulation models (pp. 4/12).

I am fascinated by this near doubling of the rolling resistance even though the vehicles should be running nearly identical tires. Some of the rolling resistance may come from transaxle internal drag and this is something I'm especially interested in. Regardless, this is a substantial difference in rolling resistance and I'm am really curious where these numbers came from.

#3 - Engine Power BSFC

Figure 5: Optimal fuel Economy-Line (pp. 5/12)

The Atkinson engine line only goes to just over 43 kW, consistent with the NHW10 model. But we know the later Prius engines actually generates power out to 52 kW (NHW11) and 57 kW (NHW20.) I'm finding some interesting things happening above the 4,000 rpm limit of the NHW10.

Based upon the Chief engineer's comment about using the 1.8L engine, I'm beginning to think the BSFC roll-off that seems to start at 2,600 rpm may be from the increased drag that comes from spinning faster and faster. It has a linear decrease that seems to be proportional to rpm right up to 4,150 rpm, when something bad happens.

FYI, I don't really like showing the engine speeds in Figure 4/5 as radians per second instead of the more normal 'rpm'.

#4 "During simulations it was found that at certain stationary vehicle speeds, the angular speed of the generator EM1 and the torque of EM2 changes sign. ..." (pp 10/12)

They missed the point that energy recirculation allows the Atkinson engine to turn slower. Sometimes called "heretical mode," it is one of the tricky things that lowers ICE rpm and improves performance. Overall, they've identified what is happening but their analysis is a little light compared to our knowledge.

#5 "Regarding the engine displacement and combustion cycle, the following ..." (pp. 11/12)

Their write-up makes the Atkinson cycle sound better than the diesel but I would like to know a little more about the source of the BSFC measurements (or were these just simulated BSFC?) As a Prius owner, I'm biased towards "225 g/kWh" (pp. 10/12) as being not just good enough but 'prefect' but I also realize there is more to engine efficiency than just one number or a few graphs.

If they are modeling the engines, I think they may be able to incorporate mechanical drag, an energy loss proportion to the rpm, into the model and give us a better idea of what is going on. But if these are coming from some source, I would really like to understand more about where it came from.

Over all, I'm impressed with this paper. Some of the translations are a little awkward but it looks to have covered the basics pretty well. I would look forward to their follow-up work.

Anyone know when the paper was published?

Bob Wilson

 

Yes it is but a greater force on the piston pushing it down. The losses in this case are smaller than the losses of pushing air into and sucking it out of the inlet manifold. It's a bit like pushing a spring, almost every bit of what you put in you get back out although there is a small heat loss. There are some frictional losses at the pistons and rings but not a great deal.

You have no doubt heard the noise a diesel truck makes when slowing down? This is the Jacobs Brake, an ingenious device which allows the piston to compress air in the cylinder thus doing work but at the top of the compression stroke the exhaust valve opens releasing that compressed air to atmosphere preventing it from pushing the piston back down thereby slowing the vehicle.