Why no diesel Prius?

[bwilson4web]

Originally Posted by Christ

Apples to Apples, the gasoline engine currently in use by the prius also doesn't have regenerative braking. The electric motor stored in the housing on the back of the gasser's block is what creates that function.

We call that "gasser's block" the transaxle but I'll adopt your curious term, "the back of the gasser's block."

Originally Posted by Christ

. . .
Diesels aren't inefficient at low power, they're inefficient at low LOAD... same thing with gas engines. . . .

Both a diesel and a gas engine require a minimum amount of fuel burn to run at idle. Call this the 'heartbeat' or minimum operating fuel burn rate. If the engine, diesel or gas, requires double that amount of fuel to provide the "low LOAD," the efficiency has increased over idle, a little bit. But the Prius plays a trick with its "the back of the gasser's block" (called a transaxle.)

The Prius produces much more energy than is needed by the load so the engine 'hearbeat' or minimum idle burn rate is a small fraction of the total fuel burn. The energy in excess of what the vehicle needs is converted into electrical energy stored in the battery. Then soon as there is enough energy in the battery, the Prius gas engine shuts down and the stored energy in the battery is used to move the car. Thus a larger portion of the Atkinson cycle energy is ultimately used for vehicle power compared to the diesel that is running in a power range that a significant portion of its energy just meets the 'idle power' need.

In contrast, the diesel-only car at "low LOAD" continues to pay a significant portion of its fuel burn just to keep the engine running. Without an integrated "the back of the gasser's block" (called a transaxle,) the diesel performs poorly at "low LOAD" (aka., city driving.)

So what do you call a diesel transmission or transaxle, some sort of "the back of the diesel's block?"

Originally Posted by Christ

. . . Diesels can be run as low power as you want them, provided the engine's dynamic displacement compares to the power being created.

The problem is the minimum energy needed at idle remains a significant portion of the energy burned just to keep the engine turning over, the 'idle fuel burn.'

Now part of the internal friction comes from the piston rings having to seal the gas pressure. On the compression stroke, the diesel goes from 20-to-1 whereas the Atkinson engine Prius goes from 8-to-1. The Prius has a lower pressure load on the rings than the diesel. Then comes the power stroke and both are dealing with similar pressures and friction.

Originally Posted by Christ

. . .(This is where EGR comes into play.)

At high power settings, the 1.8 Liter Prius engine uses excess EGR flow to fill the combustion chambers, which decreases dynamic displacement, keeping BSFC higher by lowering cylinder temps.

All I know is what I've read in SAE 2009-01-1061, "Development of New 1.8-Liter Engine for Hybrid Vehicles" Kawmoto, Naiki, Kawai, Shikida, and Tomatsuri, Toyota Motor Corporation:




I didn't see them using "dynamic displacement" to explain what is going on. Is that some sort of diesel term?

Originally Posted by Christ

. . . Diesels don't have difficulty handling start/stop operation as much as people like to think. Talk to a farmer some day, who runs diesel tractors.

Usually it takes a little more torque and energy to turn a crank against a 20-to-1 compression stroke than an 8-to-1 compression stroke for identical displacement engines. Having cranked a Model A Ford before, I remember the effort needed.

Originally Posted by Christ

. . . As far as being heavy, compare the power to weight ratio (and potential power to weight) of a diesel to a gasser... they're about the same. The diesel has the potential to produce the same horsepower at the same weight (meaning smaller displacement) than a gasser.

<sigh>Yes of course, the 'potential' is always there.

Bob Wilson

[bwilson4web]

Originally Posted by alohaspirit

. . .
R.I.P. Golf Turbo-Diesel Hybrid. We hardly knew ya. . . .

I don't think we have the final word, yet. It is becoming difficult for non-hybrid companies like VW to ignore the Prius sales and the continued performance improvements. Sad to say, they still don't understand the engineering.

I'm afraid we are likely to see 'greenwash' hybrids ... large fuel engines with tiny electric motors. This was the secret to GM's success ... 'greenwash'.

Bob Wilson

[Christ]

Originally Posted by bwilson4web

We call that "gasser's block" the transaxle but I'll adopt your curious term, "the back of the gasser's block."

I mistakenly thought that the e-motor was placed in a housing between the transaxle and engine, thus being "on the back of the gasser's block" I'm well aware of the terminology associated with automotive engineering, thank you.

Both a diesel and a gas engine require a minimum amount of fuel burn to run at idle. Call this the 'heartbeat' or minimum operating fuel burn rate. If the engine, diesel or gas, requires double that amount of fuel to provide the "low LOAD," the efficiency has increased over idle, a little bit. But the Prius plays a trick with its "the back of the gasser's block" (called a transaxle.)

The diesel is always fundamentally operating under less load than any throttle controlled gas engine, due to the lack of intake restriction, making for less pumping losses. Frictional losses are slightly increased due to cylinder temperature differences between the diesel and (significantly cooler) gasser.

The Prius produces much more energy than is needed by the load so the engine 'hearbeat' or minimum idle burn rate is a small fraction of the total fuel burn. The energy in excess of what the vehicle needs is converted into electrical energy stored in the battery. Then soon as there is enough energy in the battery, the Prius gas engine shuts down and the stored energy in the battery is used to move the car. Thus a larger portion of the Atkinson cycle energy is ultimately used for vehicle power compared to the diesel that is running in a power range that a significant portion of its energy just meets the 'idle power' need.

What does this have to do with any reason that a diesel couldn't be used in the same hybrid system? Remember, way back there, when noone ever said that it should be diesel only? Yeah, I actually pointed it out a few posts back... that it could still retain the hybrid system, and still be more efficient than the gasser version. I don't suppose you've ever seen that the most fuel efficient Prius runs actually involved not using the e-motor at all...

In contrast, the diesel-only car at "low LOAD" continues to pay a significant portion of its fuel burn just to keep the engine running. Without an integrated "the back of the gasser's block" (called a transaxle,) the diesel performs poorly at "low LOAD" (aka., city driving.)

Prove it.

By the way, Low Load situations DON'T include city driving. City driving is considered variable high-load driving, since you're always either accelerating or braking (or, in the case of many here, coasting.) Low load driving is on the highway, maintaining a steady speed, where the horsepower requirement to cruise is a low comparison to the potential output of the engine at the given engine speed.

So what do you call a diesel transmission or transaxle, some sort of "the back of the diesel's block?"

Assuming that there was an e-motor attached to a housing behind the diesel engine, I would have used the same terminology. Thank you for attempting to make light of something you obviously didn't understand.

The problem is the minimum energy needed at idle remains a significant portion of the energy burned just to keep the engine turning over, the 'idle fuel burn.'

Except the quite obvious fact that a diesel, while possibly burning more fuel to idle (although highly unlikely, given the nature of diesel engines), can also idle significantly lower than the average gas engine, and do so reliably, thus cutting that fuel consumption. While the consumption per revolution might be higher, the number of revolutions per minute and thusly, fuel used per minute will still be lower.

Now part of the internal friction comes from the piston rings having to seal the gas pressure. On the compression stroke, the diesel goes from 20-to-1 whereas the Atkinson engine Prius goes from 8-to-1. The Prius has a lower pressure load on the rings than the diesel. Then comes the power stroke and both are dealing with similar pressures and friction.

That may be, but higher compression engines extract more energy from each revolution as well.

All I know is what I've read in SAE 2009-01-1061, "Development of New 1.8-Liter Engine for Hybrid Vehicles" Kawmoto, Naiki, Kawai, Shikida, and Tomatsuri, Toyota Motor Corporation:

[Images Omitted]

I didn't see them using "dynamic displacement" to explain what is going on. Is that some sort of diesel term?

No, it's an engine design term. Static displacement is a measure of an engine's available capacity. Dynamic displacement accounts for volumetric efficiency. Volumetric efficiency is a comparison of actual drawn volume versus Static Displacement, expressed as a percent. For example - A 1 liter single cylinder engine running at 75% VE is drawing/expelling .75 liters per intake stroke, or .375 liters per revolution. Volumetric Efficiency is based on air/fuel mix, regardless of air/fuel ratio. Adding EGR flow to the cylinder during operation changes the Dynamic Displacement Availability, but does not affect VE when expressed as a percent of ACTUAL available capacity. Understand now?

Usually it takes a little more torque and energy to turn a crank against a 20-to-1 compression stroke than an 8-to-1 compression stroke for identical displacement engines. Having cranked a Model A Ford before, I remember the effort needed.

Sure, it does. That's obvious. The extra weight of the internal components also requires more work to move in a diesel. It also takes more energy to pump diesel fuel at the high pressures necessary for proper operation in a diesel engine. Petro-Diesel fuel also contains ~15% more energy per unit than gasoline, and Diesel engines are better at extracting energy with less wasted energy than gasoline engines.


<sigh>Yes of course, the 'potential' is always there.

Potential, in this case, isn't the unrealized kind that you seem to think it is.

Bob Wilson

Christ Pettitt. Nice to meet you.

.

[bwilson4web]

Hi,

I would recommend getting a copy of SAE 2009-01-1061, "Development of New 1.8-Liter Engine for Hybrid Vehicles" Kawmoto, Naiki, Kawai, Shikida, and Tomatsuri, Toyota Motor Corporation, and SAE 2009-01-0726, "Development of New Hybrid Transmission for Compact-Class Vehicles" Mizuno, Ibaraki, Kondo, Odaka, Watanabe, Mizutani, Kaneshige, and Kitada, Toyota Motor Corporation. The first fully covers the new 1.8L engine and the later, the transmission. However, these are not 'systems views' of the 2010 Prius and many times, a general discussion can help clear up some misconceptions.

The Prius engine 'cheats' the pumping losses by using an Atkinson cycle that keeps the input valves open during half of the compression stroke. Such simple words belie what is going on. It is an intake valve trick that significantly reduces pumping losses seen in an Otto engine but requires doubling the number of phases involved to understand:

Phase angle - Otto cycle - Atkinson cycle
0-45 - power stroke - power stroke
45-90 - power stroke - power stroke
90-135 - exhaust stroke - exhaust stroke
135-180 - exhaust stroke - exhaust stroke
180-225 - intake stroke (pumping loss) - intake stroke (pumping loss)
225-270 - intake stroke (pumping loss) - intake stroke from adjacent cylinder (no pumping loss)
270-315 - compression stroke - push charge to adjacent cylinder (no pumping loss)
315-360 - compression stroke - compression stroke, half charge

The Atkinson cycle trick occurs at angles 225-315 when the intake valves of two cylinders, one beginning compression and a second beginning intake, where the charge passes from the first cylinder to the second cylinder. There is no 'pumping' loss during this ~45 degree phase. But the variable intake valve also replaces part of the traditional throttle plate function.

Now if you'll re-read page 5 from SAE 2009-01-1061, the 1.8L engine paper, you'll notice the intake valve has a variable range of 41 degrees. This change in intake valve opening replaces some of the throttle plate by controlling how much fuel-air leaks from one cylinder to the next. In a traditional Otto engine throttle, all of air intake has to push through that narrow gap. In contrast, the Atkinson cycle is just passing part of the charge from one cylinder to the next without the throttle plate, narrow gap. But these are details of implementation whose real impact shows up in vehicle performance.

The bottom line, "the proof," is in the mileage numbers from an engineering lab, like the EPA requires, as found at www.fueleconomy.gov:

City - Highway - vehicle engine
51 - 48 - 1.8L, 2010 Prius
29 - 40 - 2.0L, 2009 Jetta TDI

The outstanding Prius mileage is a product of the Atkinson cycle engine and efficient transmission/transaxle combined with a sophisticated set of control laws. One's understanding or lack of understanding of these Prius systems doesn't really matter when standing by the fuel pump and paying the bottom line.

I am reminded of the story of the aerodynamic analysis of the bumble bee that proved they could not fly. Yet there they are, buzzing around. It is possible that the problem was not the bumble bee but the limited, aerodynamic analysis. There comes a time when old concepts have to be replaced by improved understanding.

So let's assume that the starting motor power is the amount of energy needed to spin an engine at idle. This is a fair approximation of the minimum energy losses of each:

• 20-to-1 diesel - the high compression resistance requires a lot of torque, more power
• 8-to-1 Atkinson - the low compression resistance requires less torque, less power

Meanwhile, the overlapping, open valve of the intake and compression stroke further reduces the 'idle' fuel burn but the Prius has one more trick ... it turns off the engine. Unlike our diesel friends, the Prius turns the engine off and runs on the stored traction battery energy until more power is needed.

These are not easy concepts to master, especially after a life-time of dealing with traditional Otto and diesel cycle engines and traditional passive transmissions. I'm hoping our diesel friends might get a clue. If they do, good. If not, no matter, the EPA numbers are accurate.

Bob Wilson

[Christ]

You're also still not comparing apples to apples. I've read both of those papers before.

You're comparing a gas/electric hybrid to a "clean diesel" which is not a hybrid. The city efficiency of the Prius comes from the e-motor driving it when power from the gas engine isn't necessary.

If you want to compare apples to apples, you need to compare the Prius's gas engine ALONE to the Jetta's "Clean diesel", no hybrid action at all.

I'm also very familiar with the Atkinson Cycle engine and it's concepts. Saying that there are no pumping losses while the engine is pushing air back out of the cylinder and into the plenum so that another cylinder can draw it in is completely false. Yes, there are less pumping losses, but they're not gone.

Instead, what you now have is an engine that is pumping air back into the plenum, already mixed with fuel, so that another piston's intake valve can open, draw in under vacuum the pre-mixed air and fuel, with a little more air, and a little more fuel, so that cylinder can pump some of that mix back into the plenum, so another cylinder.... blah blah blah. The engine really isn't that hard to understand. Comp Cams built a package for race engines in the 70's and 80's that allowed people to run 19:1 compression on leaded gas, which followed that same principal. This is also where dynamic compression and dynamic displacement come into play.

19:1 static compression means literally nothing when designing an engine. You can build an engine to a MUCH higher tolerance than that, drawing in a full cylinder (100%VE), as long as you're not compressing a full cylinder. This is where the Atkinson Concept camshaft comes into play. If the camshaft allows part of the air charge back out of the cylinder, you can run an insanely high compression ratio, with better efficiency per unit of fuel used, than a traditional Otto Cycle engine.

So sure, your Toyota's Atkinson Concept engine (It's still not a true Atkinson Cycle engine) might have less pumping losses per unit used, but it's also making MUCH LESS power per pound (both pounds of engine weight, and pounds of air drawn) by having to process that air in a non-linear fashion, several times, without actually making any power from it.

Guess what? Diesel engines could use (probably not benefit from, but use) an Atkinson Concept camshaft design as well.

[tjts1]

Diesel prius would be stupid expensive and diesel engines have trouble meeting emissions regs when the engine is starting which a hybrid does a lot of. Besides, gasoline engine is cleaner, qieter, lighter and much cheaper to build. I would rather they invest more money into the electric side instead of the ICE. By the way, the prius already uses an atkinson cycle engine. Look that up some time.

[alohaspirit]

Originally Posted by tjts1

Diesel prius would be stupid expensive and diesel engines have trouble meeting emissions regs when the engine is starting which a hybrid does a lot of. Besides, gasoline engine is cleaner, qieter, lighter and much cheaper to build. I would rather they invest more money into the electric side instead of the ICE. By the way, the prius already uses an atkinson cycle engine. Look that up some time.

Diesel prius would be stupid expensive = Yes

diesel engines have trouble meeting emissions regs = No

Besides, gasoline engine is cleaner, =

qieter, lighter = A little

and much cheaper to build = Depending

I would rather they invest more money into the electric side instead of the ICE. = Do you drive an electric car?

[bwilson4web]

Hi,

I'd hold off on calling anything "stupid" but rather point out that engineering often entails trade-offs to find if not an optimum, at least an acceptable solution. I was hoping to make sure we're all on the same sheet of paper with similar understanding and that seems to be the case. Then comes the detailed analysis.

In words, we've described all of the elements (hopefully for lurkers who may not have understood some the fine details.) The next phase would be to load this knowledge into a model and run the simulations. Will there be a compression-ignition, hybrid vehicle in the future, I don't know. There isn't one in production, yet, but that might change.

I note that the chairman of VW in the first week of July talked as if they are going to move towards vehicles with a significant, electrical motive capability. He correctly identifies this as a long-haul goal and my best guess is:

• 2-3 years - engineering design time
• 1st generation - needs about 3 years of road time for 'lessons' learned
• 2nd generation - fixes worst, still has some problem areas, 3-5 years
• 3d generation - what the 2010 is today

Right now, I can't get excited by the current VW diesels, their metrics don't work. But VW may have seen 'the writing on the wall' and it will take them a little time to get something designed and in the market place. I just hope they don't follow the GM model and make "greenwash" hybrids whose electrical parts are little more than glorified alternator/starter motors.

Bob Wilson

[alohaspirit]

Its a really hard market for the masses,
almost like trying to tell people to skip Mcdonalds and drink more clean water

probably the story of our life


which is why the Gen1 Insight died
no one wanted a small 5 speed 2 seater

same reason why small electric cars dont take off
because people dont "feel" safe



Honda copied the Prius due to mass appeal

Tesla followed suit

Conclusion:
Most people want Prius-size cars first, upgrades second, cost third, global impact last

[rmay635703]

Originally Posted by bwilson4web

Usually it takes a little more torque and energy to turn a crank against a 20-to-1 compression stroke than an 8-to-1 compression stroke for identical displacement engines. Having cranked a Model A Ford before, I remember the effort needed.


<sigh>Yes of course, the 'potential' is always there.

Bob Wilson

More torque but less duration, most diesels will start in one single compression cycle once warm, I do not know of any gasoline motors that can do that repeatably.

Originally Posted by tjts1

Diesel prius would be stupid expensive and diesel engines have trouble meeting emissions regs when the engine is starting which a hybrid does a lot of. Besides, gasoline engine is cleaner, quieter, lighter and much cheaper to build. I would rather they invest more money into the electric side instead of the ICE. By the way, the prius already uses an atkinson cycle engine. Look that up some time.

Your statements are mostly false, my 1982 diesel suburban once warm starts nearly instantly and emits no additional exhaust during warm startup than normal. It always amazed me that I could turn the key into start and back out as fast as I could move it and the motor always started fine. Truth be told there have been instant on/off diesel prototype cars floating around since the 80's most could start in ONE compression cycle unlike a gasser that always needs some degree of cranking duration. A simple spring was used if I remember to kick the motor off and on rapidly.

Diesel emissions are much more cleanable than gas emissions, lest we forget mining operations almost always use DIESEL equipment underground with a water wash on the exhaust because gasoline emissions are not easily made NON Lethal in enclosed spaces. On a diesel, All you end up with is a reduced amount of CO2 and water for air born emissions as compared to cleaned gas emissions which always include Carbon monoxide. Why people are unwilling to make a bit of fertilized water (which can be recovered and reused ONBOARD electrically mind you) on their diesels is beyond me, diesels DO NOT need to emit any airborne emissions besides CO2 even with high sulfur fuel with a simple water or saltwater wash. True fertilizer and water use is irritating but considerably less than smog I would guess?

Another word of note is that Diesel engines can be built for about the same price as gasoline engines, they just are never marketed that way (aka artificial inflation, lower demand). The real price difference in labor and materials is trivial. Diesels DO NOT need to be as complex as they are being made, to compete with a gasser, the emissions non-issue is what is making the problems. And the solution for diesel emissions has been known since the 40's, sadly only ocean going vessels and miners see the solution.

Cheers
Ryan