2 stroke thread (with occasional F1 relevance!)

[gruntguru]

Reasonably, together with the SNR another number should be given indicating the variation of the combustion torque versus the average torque provided.

Yes. It seems Furasawa did consider only the inertia torque as "noise" however, I agree with the need for another metric (similar to yours) which considers the ratio of all alternating components of torque (AC) to the average torque (DC). When the rear tyre feels an oscillation in the applied torque, it cannot tell whether that oscillation is due to inertia torque or combustion pulses.

[J.A.W.]

Here is a current production 2T unit showing useful data: http://www.recpower.com/3703%20-%203%20 ... 0cycle.htm

[manolis]

Hello J.A.W.


Here is a UL260 for comparison:



The basic version provides 97HP@3,300rpm and weighs 72.3Kg (including all accessories, oil and exhaust).

Price: 16,200Euro (US18,000$).

2,590cc capacity (105,6mm bore, 74mm stroke, boxer, 8.16:1 compression ratio).

Specific fuel consumption with full throttle:



which gives 310gr/kW/Hour (which means 27% Break Thermal Efficiency at peak power).

Direct drive of the propeller.

Air cooled (at UL-power they are proud for their air-cooling).

Four stroke.



The two-stroke three-in-line Hirth 3703 with the necessary “reduction speed” unit (but without the radiator) weighs 58Kg (i.e. more than 80% of the weight of the big four-stroke).



The required reduction unit “consumes” some of the 100HP of the Hirth 3703.

Including the “Speed Reduction Unit” and the “Oil Injection Option”, the price of the Hirth 3703 is US14,500$ (80% of the big four-stroke).

The Hirth needs a counter-rotating 1st order balance shaft to reduce the heavy free inertia moment (in comparison, the boxer UL260 is way better as regards its vibration free quality).
However there is no counter-rotating balance shaft in the Hirth 3703; this makes necessary special mounts for the engine and stronger / heavier airplane frame.



At peak power the mean piston speed of the UL260 is only 8.14m/sec while at peak power the mean piston speed of the Hirth 3703 is 13.8m/sec (70% higher).
The lower mean piston speed is a big advantage for the reliability and the TBO.


The exhaust emissions is another big issue.


So, I can’t see true advantages of the conventional two-stroke Hirth 3703 engine.

Thanks
Manolis Pattakos

[J.A.W.]

Hi Manolis,

Well, sure - apart from the bulk, cost, having to utilize 2.5 times the displacement & many more moving parts..

& the even firing 120` 2T triple is bound to be both smoother, & be more easily/ conveniently mounted.

Which of the two would make the more useful basis for a motorcycle power unit?

[manolis]

Hello J.A.W.

Despite the 2.75 times higher capacity and the “many more moving parts” of the four-stroke, the two-stroke propulsion unit reduces the weight less than 20%.
Isn’t it interesting?


I couldn’t find any official data for the specific fuel consumption of the three cylinder Hirth 939cc.

The only they mention is: “3703 fuel consumption rivals similar Hp 4 cycle engines”.

Hirth should be more serious about the fuel consumption which is a big issue in the 2-strokes.
Why?
Because the specific fuel consumption greatly affects the overall cost (ownership cost plus running cost) of the engine.

For similar power (say 80% of the peak power at cruising) the UL260 runs at substantially lower mean piston speed.

Supposing a 20% higher specific fuel consumption for the high-revving 2-stroke (it is way more in the case of the 3703 with the carburetor), for a 3 hours flight with 24l/hour (24=0.3liter/hp/hour * 80hp) it makes another 10Kg of fuel at take-off, which means same overall weight with the big and bulky four-stroke.

Suppose 1,000hours TBO for both.

In the time between overhauls, the two-stroke consumes 4,800 liters more fuel than the four stroke (in Europe, wherein the fuel is more expensive, it covers by far the price difference of the two engines, in the USA it covers more than 80% of the engine price difference).


The smaller intervals between combustions is an advantage of the Hirth 3703, however the unbalanced first order inertia moment is a big issue. Adding a balance shaft means more weight, more friction and higher cost for Hirth.


For an airplane the UL260 seems, from various viewpoints, a better choice than the Hirth 3703.

For a motorcycle, the bulky 4-stroke UL260 is worse than the 2-stoke Hirth 3703.
But this doesn’t make the Hirth 3703 a good choice for a motorcycle.


What I say is that the conventional 2-stroke has lost the game.

In order the 2-stroke to revive, fundamental changes are required.


Like those proposed by Piero Baldini (who spent his career designing engines for Piaggio / Vespa) with his Primavis engine:




Or like those changes proposed by Rotax / Mercury with their E-TEC technology.





The photos are from http://www.snowmobile.com/manufacturer/ ... -850-e-tec .
They would be better if the photographer had disconnected / put aside the cables.

Thanks
Manolis Pattakos

[J.A.W.]

Hi Manolis,

A 120` 2T triple has a natural "rocking couple" - but at the rpm the Hirth is doing, the actual effect is insignificant.

I have an 800cc 2T triple solidly mounted in a motorcycle chassis - in which vibration only becomes intrusive above 7,000 rpm.

Incidentally, this very basic piston-port Kawasaki engine gives a best consumption figure: hp/hour of ~300 grams, @ max out put
- as good as the 4T..

The Hirth triple link listed above - does show fuel consumption figures - on the power chart.

Obviously, a significantly higher specific out put engine must be more robustly constructed,
& that 4T engine is very 'lazy' by comparison, & so can get away with being built light, even if perhaps
at the cost of being a bit flimsy & prone to heat-soak/power-fade, when used hard.

The "game" as shown by the VW "cheating" scandal, may be subject to rule changes,
- whereas the laws of physics - which favour 2T piston engine function, are not..

[manolis]

Hello J.A.W.

You write:
“The Hirth triple link listed above - does show fuel consumption figures - on the power chart.”

Thanks.



Provided the Gal/Hr are US Gallons, and taking the specific density of the gasoline fuel as 0,75gr/cc, the BTE (Break Thermal Efficiency) of the Hirth 3703 at 5,000rpm / 80HP is calculated at 35%.

It is too good to be true.
The two-stroke Hirth 3703 appears having higher BTE than many reputable Diesels.

At peak power the BTE of the 3703 drops at 27% (i.e. as in the UL260 four stroke).


A reasonable explanation: the power chart of Hirth refers to UK gallons.

With the Gal/Hr being UK Gallons, the peak Brake Thermal Efficiency of the Hirth 3703 drops to the more reasonable 29% (which is still high) and the BTE at peak power drops to 22,4% (which gives 18% higher specific fuel consumption than the big capacity UL260 four-stroke, more or less as the assumption in my previous post).

But again, in the same page with the above graph all the prices of Hirth are in $, not in £.

Any idea?

Thanks
Manolis Pattakos

[J.A.W.]

Hi Manolis, the graph shown is on a US site, & since Hirth Germany would be metric, it is likely, US gal.

Note that the synergistic-harmonic 120` triple pulse capability appears to be utilized by the close-coupled exhaust group.

So, those steady state efficiency numbers are not altogether unlikely.. a check with the Hirth site may confirm it,
(or reveal it - as a transcription error).

FYI, Mercury uses the Orbital 'Optmax' DI system, whereas BRP/Rotax/Evinrude have the E-TEC DI.

[manolis]

Hello J.A.W.

You write:
“So, those steady state efficiency numbers are not altogether unlikely.. a check with the Hirth site may confirm it,
(or reveal it - as a transcription error).”


The 35% BTE for a conventional two-stroke (like the Hirth 3703) is too good to be true.

In order Toyota to achieve 36% Brake Thermal Efficiency with their PRIUS engine, they had to reduce the power output by one third, they had to use “late intake valve closing” (Atkinson / Miller cycle), they had to increase the geometrical compression ratio at 13:1 (over-expansion), they had to reduce the number of the main bearings of the crankshaft at only three (from the normal five), they also had to apply every trick they new to milk more energy from the fuel.

And this 36% BTE of the Toyota PRIUS engine was achieved in a narrow revs / load range.

Obviously Hirth gives the wrong data (even if the Gallons are UK-Gallons, the BTE is too high).

At http://www.rotec.com/2-stroke-power-torque-fuel the specific fuel consumption of the “famous” two-stroke Rotax 582 is given as 410gr/kW Hr (which means 20% BTE).
From a conventional two-stroke like the Hirth 3703 I would expect something similar.



You also write:
“Mercury uses the Orbital 'Optmax' DI system, whereas BRP/Rotax/Evinrude have the E-TEC DI”


In case the data at http://www.boat-fuel-economy.com/mercur ... ion-liters are true, here is the calculated BTE of the various Mercury Optimax two-stroke engines:



Despite they are using the famous Orbital DI system, they are not good, at least not at full power.



Similarly for the Evinrude E-TEC marine engines:

Take the small Evinrude ETEC with the 50HP. With 17.2L/Hr official fuel consumption (WOT) it gives a BTE of only 23%.

Take the big Evinrude ETEC with the 300HP. With 97.9L/Hr official fuel consumption (WOT) it gives a BTE of only 24%.

Despite the “high-tech” systems they use, the Mercury Optimax, the Evinrude E-TEC (and the Rotax E-TEC) two-strokes are not really good in practice.

The two-stroke does have advantages, but it needs fundamental changes to exploit them.



Take another look at the Power / Consumption graph of the Hirth 3703 (previous post); here is another interesting thing to think about:

The 3703 engine makes 80HP consuming 4.8Gal/Hr.
In order to make another 20HP (i.e. 25% more) it is consuming another 3.0Gal/Hr (i.e. 62.5% more fuel).
Differently speaking: provided the engine makes the first 80HP at 35% BTE, the same engine makes the rest 20HP (peak power: 100HP) at a BTE of only 14%.

Thanks
Manolis Pattakos

[gruntguru]

Manolis. Would you not also expect the WOT efficiency of the Evinrude engines to be significantly lower than the best BTE (like the Hirth)? I would be surprised if the engines run lean, stratified charge at WOT. This would be detrimental to the peak specific output.

[manolis]

Hello Gruntguru.

You write:
“Would you not also expect the WOT efficiency of the Evinrude engines to be significantly lower than the best BTE (like the Hirth)? I would be surprised if the engines run lean, stratified charge at WOT. This would be detrimental to the peak specific output.”


The peak BTE of the Evinrude engines is expected substantially higher than the BTE efficiency at peak power.
The question is: how much?

The Hirth 3703 power chart seems unreliable.
It gives 35% BTE at 80HP/5,000rpm which is too much for a conventional 2-stroke.
So it cannot be taken as a basis.

It would be useful if we had reliable data for the BSFC (Brake Specific Fuel Consumption) at partial loads.

On the other hand, a lower BSFC at the peak power (the WOT is not the correct term) combined with a higher specific power is “the” challenge for the engine makers.
The “Fuel Economy” tables in the Internet are, mostly, for the peak power.



Another interesting / strange thing is that the four-stroke engines of Honda, Yamaha, Suzuki presented in the Boat Fuel Economy site have not significantly better BTE than the two-strokes of similar power output (always measured at peak power).
The 4-strokes are better but not significantly.
Only some MerCruisers are substantially better.
For instance, the 4-stroke MerCruiser 5.0 MPI 305 cid achieves a BTE of 29.5%.



Here is the calculated BTE of some Volvo Penda Diesel four-stroke engines:



The specific density of the Diesel fuel is taken 0.83g/cc.

As the Evinrude E-TEC / Rotax E-TEC / Mercury Optimax (i.e. as the most advanced two-strokes), similarly the Diesels cannot run too lean at peak power.

However they keep a high BTE (near 40%) till their peak power.

For instance:
The 300HP Evinrude E-TEC two-stroke consumes, at peak power, 80% more fuel than the 300HP D4-300 Volvo Penda Diesel 4-stroke (3,7L displacement, turbocharged) running at peak power, too.

Thanks
Manolis Pattakos

[gruntguru]

The moral of the story is - don't cruise your spark ignition boat engine at WOT (and especially at peak power).

[J.A.W.]

Well, that depends... on how far/fast you want to move.. if 1/2 speed/power is ok, then ~3 X distance..
See:
https://www.youtube.com/watch?v=ruTJ3A-7L5E

Obviously, a heavy marine diesel of the same power rating - would def' not suit that boat.. or go as fast..

This is the home site for ( now Chinese owned) Hirth: http://www.hirth-engines.de/index.php/en/2-stroke-logic

They have data sheets available for download, so perhaps Manolis can check the metric fuel consumption figures against the gallons documented in the US data.

[gruntguru]

Well, that depends... on how far/fast you want to move..

The answer is there - in the word "cruise".

[J.A.W.]

Well, that depends... on how far/fast you want to move..

The answer is there - in the word "cruise".

& "cruise" is such a variable loaded matter, isn't it?

Parameters range from 'most economical' to 'best comfort' to 'mechanical limitation' speeds...

Back ~40 years ago when you could buy a showroom new largish engine capacity road-going 2T motorcycle,
MCI tester, Dave Minton wrote this, about the biggest capacity such machine offered, the Kawasaki H2 triple :

"The speed advantage of this machine lying solely in its immense torque spread, allowing big throttle
openings from as low as 2,000 rpm in top gear. As this is only around 40 mph, the usefulness of such a
wider power spread can be appreciated fully by riders with experience of long, fast rides when the
continual effort of gear changing becomes a wearying & sometimes painful chore.

In other words, the 750, once free of town can be held in top gear almost indefinitely without losing
ground during low engine speed acceleration.
I know full well that Vincents used to do it, & Squariels & other oldies.

What they would not do - & never did, so forget the legends - was to cruise at the tremendously
high speeds of this one.
It has no limit.
Top speed, which on test came out at around 125 mph - was also cruising speed.
If - you were man enough to withstand the combined strain on your arms, neck & wallet."

So Minton does aptly describe the excellent 2T attribute of being willing & eager to run up to whatever
power setting level maybe be required, then "cruise" there, for as long as needs must & consumables last..