Taste of IS250

[IS200]

Re: Taste of IS250

The is some lag in response when u try to push the car from stationery,

Most 6- or 8-cylinder engines demonstrate this lag - due to higher moment of inertia with all the extra metal (vs 4-pot engines). So it is not correct to compare your E90 320i with the IS250 engine.

My current 523i demonstrates that 6-cylinder lag too. So did the 323i that I test drove recently.

Some like the old 2.2L in the E46 320i had significant lag, compared to the 4-pot 318i. In that same timeframe, compared to the IS200 whose engine used lighter alloy pistons and con-rods, the BMW 6-potter's lag was actually much more pronounced. Some have known to floor the accelerator, while the 2.2L engine would take a split second to respond.

Engine manufacturers get around this problem in various ways.
Cadillacs have V6 and V8 engine designs that start off with just 4 cylinders firing, only to go up to 6 and then 8 cylinders at higher speeds. Reducing fuel consumption is the other reason.

Honda focused their VTEC designs on 4-potters even up to 2.4L (although they have V6 designs too at 3L) ... to compensate for a known fact that VTEC engines have got relatively low initial torque.

Between the V6 and the IL6, V6s will demonstrate a higher lag than the IL6. Don't forget that the V6 has got additional counter-rotating balancer shafts to turn too, while the IL6 does not.

Volkswagen in designing their VR6 engine got around this by making the V-angle very narrow ... just 11 degress, if I recall correctly. Thus VW's VR6 is thus more like an IL-6, but still a V setup nonetheless.

 

[DoggieHowser]

Re: Taste of IS250

That's why one of the things I love about my car is the simple but classy looking shift stick. Just 4 position with no funny looking train tracks gates. I'm the type that loves clean lines and as little cluster as possible. So the E90 is perfect...

Love my hidden cup holders too... Just nice to put Coke Light.

FWIW, I think the gated auto gearbox is a way to prevent drivers from getting into the wrong gear, eg Reverse when u want Park or Neutral or getting into 2 when you want D.

 

[Shaun]

Re: Taste of IS250

Most 6- or 8-cylinder engines demonstrate this lag - due to higher moment of inertia with all the extra metal (vs 4-pot engines). So it is not correct to compare your E90 320i with the IS250 engine.

This is not a rule.

Have to consider tract cross sectional area (CSA) - especially that of the intake. When you get on the the throttle, you have the displacement of the engine demanding flow through the intake tract. The minimum CSA will determine how quickly the cylinders are filled, which determines reponse time. Quick cylinder fill, quick burn ---> energy release. How much energy is released is dependent on mass burn, and with NA engines, mass burn is dependent on displacement in combination with tract efficiency which was mentioned above. So inertia holds the engine back, mass burn accelerates it .....which means with NA engines, one has to consider is the displacement-to-rotating/reciprocating-mass ratio.

In other words if you have a 2.0 I4 and you add 2 cylinders of similar geometry (hence component masses as well) to arrive at 3.0 I6, and adjust tracts to match, then there is no difference.
If the displacement gain is not proportional to cylinder number gain, then yes response will be slower.

Engine manufacturers get around this problem in various ways.
Cadillacs have V6 and V8 engine designs that start off with just 4 cylinders firing, only to go up to 6 and then 8 cylinders at higher speeds. Reducing fuel consumption is the other reason.

Described method of operation doesn't help response, or fuel consumption. As I understand it, DOD is run exactly opposite of this. Large engines at WOT are fuel efficient for the power they make. Large engines at part throttle are fuel inefficient for the power they make due to large surface area to volume ratio inherent. The cylinder de-activation therefore is at standard cruise where part throttle is run.

 

[IS200]

Re: Taste of IS250

This is not a rule.
In other words if you have a 2.0 I4 and you add 2 cylinders of similar geometry (hence component masses as well) to arrive at 3.0 I6, and adjust tracts to match, then there is no difference.

I agree its not a rule.

However if you use the same basis of 2L engines, the CSA for a 4-pot is likely to be larger, since there's a higher chance that it being an "over-square" cylinder design, compared to a 6-pot engine of the same volumetric displacement ... whose cylinders are likely to be more narrow than long. Thus for such cases like comparing an E46 318i or E90 320i (both 2L, 4-pot) and say an IS200 (2L, 6-pot), then the difference in moment of inertia due to the rotating mass will become significant ... this is compounded by the lower CSA of the IS200 engine.

 

[Shaun]

Re: Taste of IS250

However if you use the same basis of 2L engines, the CSA for a 4-pot is likely to be larger, since there's a higher chance that it being an "over-square" cylinder design, compared to a 6-pot engine of the same volumetric displacement ... whose cylinders are likely to be more narrow than long.

Cylinder geometry is never a concern in a street car moving off a line on street tires, especially in regular everyday driving. Power always exceeds the traction afforded by street tires.

By CSA, I meant intake tract CSA.. further upstream of the valves..from port to intake filter. The only situation where moving off the line is a possible restriction is high RPM launching with large displacement engines running variable tract length and CSA systems that perhaps to do not switch over fast enough. This isn't the case with normal driving, especially with street tire limitations.

Thus for such cases like comparing an E46 318i or E90 320i (both 2L, 4-pot) and say an IS200 (2L, 6-pot), then the difference in moment of inertia due to the rotating mass will become significant ... this is compounded by the lower CSA of the IS200 engine.

This isn't due to cylinder geometry. If anything long stroke narrow bores are aided by being able to get port gas velocities up quicker rather than catering to high RPM sport-operation as is common with very oversquare engines.

FWIW the comparison of feel moving off a line is hard to conduct with any transmission system other than a plain manual. Autos and other semi-manual systems are variables. I believe this might be a variable you are experiencing, along with the difference displacement-to-component-mass ratios. As mentioned, the minimum displacement for an I6 to create similar ratio, is approx. [1.5 X (displacement of I4)] due to balancer shaft difference. The 2L I6 doesn't come close to meeting this condition vs 1.8L or 2.0L I4s.

 

[Ahbengdriver]

Re: Taste of IS250

wah wah wah.....I enjoy these expert talk on engine lag You all must be engineers in yr own right !

 

[Shaun]

Re: Taste of IS250

Don't mean to suggest that displacement divided by a large number of cylinders is pure negative. Only real negative is inertia and friction at low engine speeds. There are positives but only in dedicated sport driving or racing. Reasons are outside the topic scope, but here's a link them. This is why we see small displacement divided up across many cylinders in racing (3.0L V10s, 2.4L V8s, 3.0L H16s, etc.). http://www.bmw-sg.com/forums/56889-post11.html If there's to be further discussion on this, I think it should be on that thread, or some other related thread in the right section of this forum.

 

[Ahbengdriver]

Re: Taste of IS250

Wah.....why those LEXUS SEs so rude to u huh, for u could potentially give them a deal?

Or maybe they only entertain AH BENGs like me who went down that day in slippers and tee shirt n cheapo bermudas, didnt even comb my hair[not much anyway]. Actually when I tested the IS250 the other day, I thot it didnt feel as good as the old IS200 which I also tested before, I heard they ve changed the engine right?

 

[IS200]

Re: Taste of IS250

Don't mean to suggest that displacement divided by a large number of cylinders is pure negative. Only real negative is inertia and friction at low engine speeds. There are positives but only in dedicated sport driving or racing. Reasons are outside the topic scope, but here's a link them. This is why we see small displacement divided up across many cylinders in racing (3.0L V10s, 2.4L V8s, 3.0L H16s, etc.). http://www.bmw-sg.com/forums/56889-post11.html If there's to be further discussion on this, I think it should be on that thread, or some other related thread in the right section of this forum.

Another big positive of having plus (more) cylinders for a given displacement engine (assuming all else being equal - including tranny / unsprung weight / tyre losses / incline etc.) is to be able to operate the engine at lower RPMs for any given cruising speed constant at similar gear ratios.
In layman's term: a much sweeter and smoother engine. With that, a lowered NVH too.

Agree with the negative in inertia at low engine speeds when there are more cylinders per litre ... which was the same point I had made earlier in this thread.

 

[Shaun]

Re: Taste of IS250

Another big positive of having plus (more) cylinders for a given displacement engine (assuming all else being equal - including tranny / unsprung weight / tyre losses / incline etc.) is to be able to operate the engine at lower RPMs for any given cruising speed constant at similar gear ratios.

I don't understand here. If vehicle speed is the same, and gear ratios are the same, how can engine RPM be any lower? They are mathematically tied are they not?

Agree with the negative in inertia at low engine speeds when there are more cylinders per litre ... which was the same point I had made earlier in this thread.

Yes. The condition is that displacement does not increase proportionally with number of cylinders gained.

 

[GTV6]

Re: Taste of IS250

Not sure where you got the facts on the V6 and V8 firing on 3 and 4 cylinders....it sounds illogical; there will be tons of vibration....are you refering to sequential injection??

To me the easiest way to reduce inertial is dead simple....reduce the mass of the flywheel! This is where all the inertia is really....you can reduce the mass of your stock flywheel by up to 30% without compromising on refinement and you will notice the snap response from the throttle immediately.

However, this is not possible for dual mass flywheels(as on most BMs)...only option is to change to an alloy flywheel.

Most 6- or 8-cylinder engines demonstrate this lag - due to higher moment of inertia with all the extra metal (vs 4-pot engines). So it is not correct to compare your E90 320i with the IS250 engine.

My current 523i demonstrates that 6-cylinder lag too. So did the 323i that I test drove recently.

Some like the old 2.2L in the E46 320i had significant lag, compared to the 4-pot 318i. In that same timeframe, compared to the IS200 whose engine used lighter alloy pistons and con-rods, the BMW 6-potter's lag was actually much more pronounced. Some have known to floor the accelerator, while the 2.2L engine would take a split second to respond.

Engine manufacturers get around this problem in various ways.
Cadillacs have V6 and V8 engine designs that start off with just 4 cylinders firing, only to go up to 6 and then 8 cylinders at higher speeds. Reducing fuel consumption is the other reason.

Honda focused their VTEC designs on 4-potters even up to 2.4L (although they have V6 designs too at 3L) ... to compensate for a known fact that VTEC engines have got relatively low initial torque.

Between the V6 and the IL6, V6s will demonstrate a higher lag than the IL6. Don't forget that the V6 has got additional counter-rotating balancer shafts to turn too, while the IL6 does not.

Volkswagen in designing their VR6 engine got around this by making the V-angle very narrow ... just 11 degress, if I recall correctly. Thus VW's VR6 is thus more like an IL-6, but still a V setup nonetheless.

 

[Shaun]

Re: Taste of IS250

Not sure where you got the facts on the V6 and V8 firing on 3 and 4 cylinders....it sounds illogical; there will be tons of vibration....are you refering to sequential injection??

Cylinder deactivation does not upset the engine because based on engine load, number of cylinders, bank angle if any, and firing order, how many and which cylinders to drop can be decided. It is also typically only employed at low loads where combustion loads are small anyway. The difference is minimal to the point that the driver cannot even detect it. Quite a number of the major OEMs have, and continue to, successfully run such systems.

To me the easiest way to reduce inertial is dead simple....reduce the mass of the flywheel! This is where all the inertia is really....you can reduce the mass of your stock flywheel by up to 30% without compromising on refinement and you will notice the snap response from the throttle immediately.

Flywheel held constant, the changes in rotating / reciprocating mass that occurs with adding or removing cylinders, will always exist.

 

[IS200]

Re: Taste of IS250

I don't understand here. If vehicle speed is the same, and gear ratios are the same, how can engine RPM be any lower? They are mathematically tied are they not?



Yes. The condition is that displacement does not increase proportionally with number of cylinders gained.

Simple.

For the same engine capacity, the complete cycle time between TDCs will be divided by 3 for 6-cylinder engines, and divided by 2 for 4-cylinder engines. Thus the rotational (angular) speed of the crank shaft (before the transmission gears) will be 1.5 times slower for the 6-pot.

Thus for the same speed and gear ratio, the engine RPM will be lower for 6 vs 4 cylinder engines.

 

[IS200]

Re: Taste of IS250

Not sure where you got the facts on the V6 and V8 firing on 3 and 4 cylinders....it sounds illogical; there will be tons of vibration....are you refering to sequential injection??

No, not referring to sequential injection.

But it is fact that the Cadillac engineers have designed it into their V8 cars since more than a decade ago. Firing 4, then step up to 8 - in 2 stages, as a function of RPM.

 

[IS200]

Re: Taste of IS250

Cylinder deactivation does not upset the engine because based on engine load, number of cylinders, bank angle if any, and firing order, how many and which cylinders to drop can be decided. It is also typically only employed at low loads where combustion loads are small anyway. The difference is minimal to the point that the driver cannot even detect it. Quite a number of the major OEMs have, and continue to, successfully run such systems.



Flywheel held constant, the changes in rotating / reciprocating mass that occurs with adding or removing cylinders, will always exist.

Agree with you ... that designed-in partial cylinder deactivation will not upset engine operations, nor cause any undue increase in vibration - especially at low load / RPM.

 

[Shaun]

Re: Taste of IS250

Simple.

For the same engine capacity, the complete cycle time between TDCs will be divided by 3 for 6-cylinder engines, and divided by 2 for 4-cylinder engines.

Nope, not at all. You are confusing different cylinder TDCs with TDC to TDC on an individual cylinder. Engine speed (engine RPM) is the time taken for a piston in any cylinder to go from TDC, down, and back up to TDC. This corresponds perfectly wih 1 revolution of the crankshaft.

An I4 engine will have one pair of pistons at TDC at 0 degrees. Rotate the engine and as the first pair gets to 180 degrees, the other other pair will now be at TDC. At this point, the crankshaft has only completed 180 degrees of rotation. Another 180 degrees of crankshaft rotation has to be applied until the original pair of pistons is back at TDC. This will be 360 degrees of rotation completed.

Now with an I6 engine, the intervals at which you see different pistons at TDC is 120 degrees but it does not mean that the engine has gone 3 revolutions for in 360 degrees of crankshaft rotation. 360 degrees of crankshaft rotation is still 1 revolution despite different pistons being at TDC at smaller intervals.

On any reciprocating piston engine, you pick any cylinder, wait for it to complete a round trip to original location, and with the time taken for this event to occur, you can calculate engine speed ( X revolution per unit time). There is no divided by 2, divided by 3, to be carried out.

Another simple way of thinking about it is just to put a mark on a crankshaft snout, or flywheel. Rotate the engine till it the mark is back where it was. Disregard when and how many times you saw any piston(s) at TDC. Time taken for mark to come full circle is rotational speed.

Thus the rotational (angular) speed of the drive shaft (before the transmission gears) will be 1.5 times slower for the 6-pot.Thus for the same speed and gear ratio, the engine RPM will be lower for 6 vs 4 cylinder engines.

Nope. For similar gearing, at a given speed, no matter W16, V10, or I3 or I4, engine RPM will be exactly the same. Don't confuse smoothness of delivery with reduced engine speed.

Smoothness of delivery comes from the fact that pulses per revolution are increased so torque delivery is closer to, but still far from uniform. Above very low levels, humans cannot tell the difference, and even then they tell by vibration (engine shake forces), not by feeling the corresponding acceleration pulses from the combustion pulses (100 - 900 Hz depending on engine configuration and RPM), since all rotating, reciprocating masses, the driven tires, and even the entire mass of the vehicle, tremendously dampen the torque spikes.

 

[IS200]

Re: Taste of IS250

OK me wrong there.

But the rotational pulsations are still divided by 2 (4-cyclinder) and 3 (6-cylinder) for angular phase, as well as angular speed, since for either case, a full crankshaft revolution is still 360 degrees.

Thus theoretically, if there were an infinite number of cylinders, then the torque pulses will flatten out like a turbine ... but then it will become infeasible since the moment of inertia to turn an infinite number of pistons and con-rods will be well ... infiinty. That's where the rotary engine comes into the picture.

Actually engine smoothness can really tell on idling (i.e. at < 800 rpm), when all kinematic factors are minimized.

 

[GTV6]

Re: Taste of IS250

How do the makers achieve cylinder deactivation? Thru' killing the spark or cutting the fuel? How does shutting down 3 cyls on a V6 bring down the inertia when you have the mass of 3 non-active cyls to contend with....pistons, con-rods and grudgeon pins. These things not only add parasitic mass to the system, not to mentiion the power loss due to the compression of gases! If cutting out the 3 cyls is out of fuel economy and emission goals, i fully agree. In fact, this have been deployed in the Bosch Jetronic FI systems back in the 80s....fuel is cut on the overrun (deceleration) below 2.5krpm.

Cylinder deactivation does not upset the engine because based on engine load, number of cylinders, bank angle if any, and firing order, how many and which cylinders to drop can be decided. It is also typically only employed at low loads where combustion loads are small anyway. The difference is minimal to the point that the driver cannot even detect it. Quite a number of the major OEMs have, and continue to, successfully run such systems.



Flywheel held constant, the changes in rotating / reciprocating mass that occurs with adding or removing cylinders, will always exist.

 

[Shaun]

Re: Taste of IS250

How do the makers achieve cylinder deactivation? Thru' killing the spark or cutting the fuel?

No, typically by killing valve actuation on selected cylinders via eccentric linkages or hydraulic valves collapsing lifters.

How does shutting down 3 cyls on a V6 bring down the inertia when you have the mass of 3 non-active cyls to contend with....pistons, con-rods and grudgeon pins.

No one claimed inertia reduction. The masses are still there and still moving, but you don't have surface area of deactivated cylinders through which to lose heat (thermal efficiency) and you have reduced pumping losses across the throttle body (often electronic to match the system), as well as across the inlet and exhaust ports of deactivated cylinders (zero pumping losses - sealed)

In fact, this have been deployed in the Bosch Jetronic FI systems back in the 80s....fuel is cut on the overrun (deceleration) below 2.5krpm.

Cylinder deactivation is different from fuel cut on overrun.

It is not my purpose or intent to explain cylinder deactivation in detail since evidence of its success is everywhere, the technology is well documented, and this thread is going way off topic. Feel free to search the terms "cylinder deactivation" or "displacement on demand" on SAE.org (technical papers for sale) or Google.

Cheers

==========

IS200, rotary engines whether of 3rd, 4th, or 10th(!) order geometry still will have toque peaks and valleys, just less than 2nd order reciprocating piston engines. Absolute constant torque is not a concern since there is mass and elastic damping and the human body is not sensitive to pick it up after all that damping. Engine shake forces are a separate issue and are not to be confused with engine torque oscillations.

 

[sunboy77]

Re: Taste of IS250

Went to see the IS250 today. Initial thought when I saw the car "Wow! big side mirrors!"

Haha but I kindof like the side mirrors. And the more I look at it from the front, the more I like it. Its aerodynamism was built to flow with the contour of the bonnet and the side of the car.

True, the back cabin was a little cramp, more so with the high transmission tunnel dumped in between. But the the high transmission tunnel, as one bro put it, is an inevitable characteristic of FR cars. Also, the drive shaft (is that what it is called - the long shaft from the engine to the wheels?) is built as a fully-staright rod, rather than a kinked one as we might see in other FR cars. Reason: a straight long shaft produces less vibration and thus less noise - characteristic of a Lexus. That explain the high transmission tunnel at the back cabin. I will say that the IS250 is more of a 4-person than a 5-person car, although there shouldn't be a problem seating a 5th person at the back.

However, the crampness of the back cabin didn't bother me much because the IS250 is built mainly with the driver in mind. Haha many sports car are built as 4-seaters also right (take DC5, SC430 as example)? All the buttons and controls are just at the right places. The paddle shift is neat. Yes the middle console is higher than most other luxury sedan - but it took me very little time to get used to. When it comes to cabin design and details, the IS250 will come up tops. Even the stitching of the leather seats were impressive.

There was a noticeable lag when I step on the accelerator - but only when I cornered the car and then pickup from there. Moving off was torquey and I was able to swing into another lane without much effort, even upon move-off.

Overall, I am excited to go back for a 2nd test-drive as I wasn't able to test all of the IS250's 205 bhp. Just felt that I did not have enough of the car. It looks good to be my next car, very soon.