3 cylinder engines

[jaa1aut]

One thing to be aware of regarding modular 3 cylinder is that the gearbox side crank seal is weak. Quite a few have been fixed on the DL under warranty. Not been widely reported.

I'm only aware of this being an issue on the 3, not the 4 or 6 cylinders. I'm not even sure if the seal is shared between configurations.

[creepy coupe]

jaa1aut said:

One thing to be aware of regarding modular 3 cylinder is that the gearbox side crank seal is weak. Quite a few have been fixed on the DL under warranty. Not been widely reported.

I'm only aware of this being an issue on the 3, not the 4 or 6 cylinders. I'm not even sure if the seal is shared between configurations.

Isn't that due to crank end float as a result of worm front bearings caps?

[jaa1aut]

creepy coupe said:

Isn't that due to crank end float as a result of worm front bearings caps?

Reason I'd heard was that it was due to cornering forces, as much more common (or indeed maybe has only occurred) on Mini than BMW 1 or 3 series.

I wouldn't expect endfloat in the bearing caps in a (fairly) new engine unless the bearings were made oversize?
You're right that bearings have been replaced at the same time though.

Edited by jaa1aut on Wednesday 12th July 16:53

[creepy coupe]

jaa1aut said:

Reason I'd heard was that it was due to cornering forces, as much more common (or indeed maybe has only occurred) on Mini than BMW 1 or 3 series.

I wouldn't expect endfloat in the bearing caps in a (fairly) new engine unless the bearings were made oversize?

If you do a search. You will there has been a major recall on 3 cylinder engines due to premature wear on the front bearing thrusts. Causing end float and clutch action problems

[Crackie]

My wife has a cooper 1.5..... I like the engine overall. Its torquey low down, the noise is interesting and it uses very little fuel. The standard map is boring at the top end but it is a little repressed http://www.superchips.co.uk/curves/Mini15T134Stage...

Edited by Crackie on Thursday 13th July 07:45

[HJG]

creepy coupe said:

RBH58 said:

Listen to a BMW 1.5 and it does sound surprisingly like an inline-6. Not kidding.

It will do. It uses the same phase angle of 120° It's not as naturally well balanced because no two pistons are at the top or bottom of the bores at any one time like a six pot.

If you look at the exhaust manifolds of an inline 6, you will (normally) see that there is one manifold for the first 3 cylinders and another for the rear 3. So it would make sense that an inline 3 would sound similar, but whereas the inline 6 fires every 120deg, the inline 3 fires every 240deg.

As correctly mentioned above, the inline 3 suffers from a rocking couple caused by the fact that when a piston at either end is at a dead centre (i.e. TDC or BDC) it is not opposed by another piston in the same position. For example: cylinder 1 is at TDC. This brings cyl 2 to 120 deg BTDC and cyl 3 to 240deg BTDC (firing order 1-3-2). Cylinder 1 is making a large upwards force by being at TDC. Cyl 3 is making a downwards force approaching BDC. This has the effect of pushing the engine up at the front (cyl 1 is at the front). The reverse happens when cylinder 3 is at TDC. This is known as a pitching couple. We can ignore the effects of cylinder 2 because the couple is acting around the centre line of this cylinder.

Counterweights are attached to the crankshaft as on any crank to alleviate the main bearings from excessive loading by counteracting a percentage of the rotating mass above the crank centreline. However we can use the counterweights to also balance some of the reciprocating mass (i.e. the piston and a portion of the con rod), which if fitted to the crank webs of cyl1 and cyl3, can reduce the pitching couples.

However we then have another problem. When a crankpin is at 90 deg, the reciprocating mass has no effective primary force (cosine 90 deg = 0). So our extra counterweighting is now causing a horizontal force at this crank position since it has nothing to counteract.

What we've now done is reduce the pitching couple but at the same time introduced what is known as a yawing couple.

Luckily we're not out of options since a balance shaft can be employed, driven at crank speed but in the opposite direction. The details of which are too lengthly for me to explain at this time of the night! This balance shaft can completely neautralise both primary pitch and yaw couples.

BMW use such a balance shaft in their inline 3 engines. The result should be a smooth engine.

Why is an inline 6 so smooth then? Well it is a mirrored inline 3, thus cancelling out all the
aforementioned couples.

[Mr Tidy]

HJG said:

If you look at the exhaust manifolds of an inline 6, you will (normally) see that there is one manifold for the first 3 cylinders and another for the rear 3. So it would make sense that an inline 3 would sound similar, but whereas the inline 6 fires every 120deg, the inline 3 fires every 240deg.

As correctly mentioned above, the inline 3 suffers from a rocking couple caused by the fact that when a piston at either end is at a dead centre (i.e. TDC or BDC) it is not opposed by another piston in the same position. For example: cylinder 1 is at TDC. This brings cyl 2 to 120 deg BTDC and cyl 3 to 240deg BTDC (firing order 1-3-2). Cylinder 1 is making a large upwards force by being at TDC. Cyl 3 is making a downwards force approaching BDC. This has the effect of pushing the engine up at the front (cyl 1 is at the front). The reverse happens when cylinder 3 is at TDC. This is known as a pitching couple. We can ignore the effects of cylinder 2 because the couple is acting around the centre line of this cylinder.

Counterweights are attached to the crankshaft as on any crank to alleviate the main bearings from excessive loading by counteracting a percentage of the rotating mass above the crank centreline. However we can use the counterweights to also balance some of the reciprocating mass (i.e. the piston and a portion of the con rod), which if fitted to the crank webs of cyl1 and cyl3, can reduce the pitching couples.

However we then have another problem. When a crankpin is at 90 deg, the reciprocating mass has no effective primary force (cosine 90 deg = 0). So our extra counterweighting is now causing a horizontal force at this crank position since it has nothing to counteract.

What we've now done is reduce the pitching couple but at the same time introduced what is known as a yawing couple.

Luckily we're not out of options since a balance shaft can be employed, driven at crank speed but in the opposite direction. The details of which are too lengthly for me to explain at this time of the night! This balance shaft can completely neautralise both primary pitch and yaw couples.

BMW use such a balance shaft in their inline 3 engines. The result should be a smooth engine.

Why is an inline 6 so smooth then? Well it is a mirrored inline 3, thus cancelling out all the
aforementioned couples.

Thanks for the technical input.

I have to admit I do like the sound of a triple ever since I rode a mate's Yamaha XS750 with a 3 into 1 exhaust! Then I bought a KH500 with Allspeed spannies - lovely noise!

But in a car I'd rather have more cylinders (at least 6 for now).

[RBH58]

RBH58 said:

Listen to a BMW 1.5 and it does sound surprisingly like an inline-6. Not kidding.

https://youtu.be/_xWItsdqKxk

Edited by RBH58 on Thursday 13th July 05:05

[335d]

Very interesting technical explanation. We have the 3 cylinder (116d - engined Mini Cooper D) and 6 cylinder (335d) engines in our cars. The 3 has always surprised us with its smoothness, particularly in the middle of the rev range. The 3 cylinder does have one odd characteristic though on startup - for the first second or so it feels very out of balance. Once the engine is idling, it feels fine, but the wobble on starting is a little disconcerting. I noticed this on both the petrol and diesel BMW 3 cylinder units, but it is more pronounced on the diesel. Perhaps the measures taken to balance the 3 cylinder unit are less effective at extremely low revs - below idle speed?

[Mr Tidy]

335d said:

Very interesting technical explanation. We have the 3 cylinder (116d - engined Mini Cooper D) and 6 cylinder (335d) engines in our cars. The 3 has always surprised us with its smoothness, particularly in the middle of the rev range. The 3 cylinder does have one odd characteristic though on startup - for the first second or so it feels very out of balance. Once the engine is idling, it feels fine, but the wobble on starting is a little disconcerting. I noticed this on both the petrol and diesel BMW 3 cylinder units, but it is more pronounced on the diesel. Perhaps the measures taken to balance the 3 cylinder unit are less effective at extremely low revs - below idle speed?

I think that sums it up nicely - a straight 6 is inherently balanced whereas a 3 isn't, so it needs measures to balance it and will never be quite as smooth all of the time. (So artificial like all the sound-bites, pops and bangs that current BMWs seem focussed on)!

[RBH58]

Mr Tidy said:

So artificial like all the sound-bites, pops and bangs that current BMWs seem focussed on

Gee, BMW are minor offenders compared to VAG and M-B. Their 4 cylinder cars sound like they have a bad case of wind.

[HJG]

Note that in addition to my explanation of inline 3 engine primary balance, there are also secondary pitch and yaw couples which cannot be eliminated by use of counterweights or single balance shaft

Eliminating these would require two further balance shafts driven at twice crank speed. It is seldom if ever such complications are considered worthwhile since the secondary forces are much lower. Again, an inline 6 and consequently V12 engines do not suffer from this issue.

[Mr Tidy]

RBH58 said:

Gee, BMW are minor offenders compared to VAG and M-B. Their 4 cylinder cars sound like they have a bad case of wind.

I know - it might suit the PlayStation generation but no good to me!

[Shangri La]

Recently I found a smashing 5 door Mini with the 1.5 litre engine it drove great, really smooth and accelerated well. I went to pick it up and pay for it. While waiting for it to be delivered I was told a spare part had to be fitted and waited quietly expecting nothing major. After waiting a further 15 minutes the salesman informed me it would take a further day to fit the spare part, which set the alarm bells ringing. On further questioning the technician faxed through to the sales dept the BMW recall notice dated 7 October 2016, which stated that the crankshaft thrust bearings have been known to wear and have ordered the thrust bearing to be replaced during a service for a modified updated design. On my reading the technical note it stated that any excessive wear if found, would require to a new engine to be fitted and the engine scrapped in situ. Costs for the new replacement engine are to be back charged to BMW. The 1.5 litre engine has been fitted to the Mini Cooper from 2014, meaning that my future Mini may have suffered the wear to its crankshaft thrust bearings and was not recalled for checking and replacement at its last service, the BMW Mini team sales team were unable to explain why it had not been recalled. On hearing this I refused to buy the car.

3 cylinder engines are inherently unbalanced maybe BMW may have underestimated the forces on the crankshaft resulting in the recall

[Locknut]

Shangri La said:

....any excessive wear if found, would require to a new engine to be fitted and the engine scrapped in situ. Costs for the new replacement engine are to be back charged to BMW.,,

... On hearing this I refused to buy the car.

Why? If I am reading your post correctly, the car would have come with a new engine.

[Mike335i]

Locknut said:

Shangri La said:

....any excessive wear if found, would require to a new engine to be fitted and the engine scrapped in situ. Costs for the new replacement engine are to be back charged to BMW.,,

... On hearing this I refused to buy the car.

Why? If I am reading your post correctly, the car would have come with a new engine.

Agreed, BMW identified a problem and put it right on a preventative basis. So what is the problem?

[iSore]

The problem shouldn't have been there in the first place.

You're not paying Kia prices plus 30% to have to sit in the cab of an AA truck whilst a dead nearly new car is scooped up at 3am.

[J1JPE]

Owned 2 MINI's with 3cyl 1.5L petrol B38 engine - people are often surprised by the burbles from outside but refined yet sporty engine notes inside. Economy good and with the auto gearbox, panel filter it goes like stink.

Same engine in our X1 Sport auto, much bigger, heavier car but BMW have mapped it accordingly; so smooth & refined but picks up speeeed well where needed. Again added only a Pipercross panel filter to aid breathing. Regularly get c.50mpg.

Reliability may be an issue longer term but first MINI lasted 4 years with no issues, mainly short journeys, 13,000 miles total only though. X1 is now over 13,000 and is 3 years old.

If concerned buy a Warranty Direct policy.

[nickfrog]

J1JPE in June 17 said:

Yes

It only took you 3 and half years to qualify your first answer!!!