Why modern diesels have lower compression ratios?

[dragvorl]

The old VW Golf mkII had a 1.6 diesel with a 23:1 CR. Modern diesels have a much lover CR around 16 to 17:1. The old ones were mainly without a turbo, of course, but anyway, that's a sharp drop in CR hence efficiency. Why are they doing that?

[NTS]

Because if you want a high boost you need a lower CR? The high boost from the turbo will bring back the efficiency at partial throttle which is the most important for most eco-labeled roadcars since the testing cycle is not a full throttle sprint.

[AnthonyG]

How do you actually give a diesel engine its compression? I mean diesel combust out of it's own under pressure, so how can you mandate that pressure?

[Lycoming]

How do you actually give a diesel engine its compression? I mean diesel combust out of it's own under pressure, so how can you mandate that pressure?

What do you mean? They're discussing compression ratio, which is determined by geometric properties of the combustion chamber.

[ian_s]

the lower static compression ratio makes them run a lot smoother and less tractor like at low revs.
this is possible due to the improvements that have been made with fuel injection, so the high compression isnt as important as it used to be. with the lower compression ratio also a lighter flywheel can be used.

i think though that the main reason is the high levels of turbo charging used these days, as NTS already said

[Holm86]

Mazda's Skyactiv-D engine is way down at 14:1 CR. Thats very low for a diesel. Their Skyactiv-G is also at 14:1 CR which is very very high for a gasoline engine.

[Pieoter]

Could it have something to do with the current trend (at least a trend in my mind) to run engine with higher lift and lower durations cams for what I assume is emisions purposes?

[AnthonyG]

How do you actually give a diesel engine its compression? I mean diesel combust out of it's own under pressure, so how can you mandate that pressure?

What do you mean? They're discussing compression ratio, which is determined by geometric properties of the combustion chamber.

Well from what I understand, diesel combusts out of it's own in the combustion chamber, so when the piston goes up and compresses the air+ fuel untill it combusts out of its own.
Now how can you then manage the ratio air-fuel, avoiding it to combust prematurely? Since in a petrol engine the fuel has properties that make it "stable" and a spark plug to ignite the combustion.

[ian_s]

How do you actually give a diesel engine its compression? I mean diesel combust out of it's own under pressure, so how can you mandate that pressure?

What do you mean? They're discussing compression ratio, which is determined by geometric properties of the combustion chamber.

Well from what I understand, diesel combusts out of it's own in the combustion chamber, so when the piston goes up and compresses the air+ fuel untill it combusts out of its own.
Now how can you then manage the ratio air-fuel, avoiding it to combust prematurely? Since in a petrol engine the fuel has properties that make it "stable" and a spark plug to ignite the combustion.

diesel combusts due to the heat created during the compression stroke. the fuel is only injected at the point when the conditions are right for the best combustion (in theory). this is very different to a normal petrol engine where the fuel is introduced before the compression stroke begins.
i think where you are going wrong is that you think a diesel engine compress the air and fuel at the same time. it doesnt, it compresses the air which makes it very hot, then injects the fuel at the right point, which spontaneously combusts.

[hpras]

From an unlikely source
http://www.economist.com/blogs/babbage/2013/07/diesels

[Tommy Cookers]

high CR in those early engines created problems for VW with excessive wear and blow-by, so they lowered CR (said at the time)
high CR also increases frictional losses, as the piston ring contact load must be higher
so car diesels don't have much more CR than is needed for self-ignition

anyway calculations as taught and in textbooks give an exaggerated estimate of the efficiency benefits of high CR
(modern modelling methods are more realistic)
the car diesel gets most benefit from partial power running without throttling not from higher CR (than gasoline engines)
increasing the mass of air and of fuel by turbocharging is limited by internal temperatures eg reducing piston life
increasing CR does the same, but is less beneficial overall than is turbocharging

this topic has been mentioned on this site already - try a search

[neilbah]

The ratio of air to fuel (AF ratio) in a diesel engine is variable since diesel engines can run well within a large range. As a very rough estimate, a tdi runs between 15:1-50:1 AF, being very smoky at the rich end. This is because a diesel is throttled by fuel, not air.

As has been mentioned i believe this is down to the injection systems, diesel fuel injection was originally mechanical and so it was somewhat crude in the timing/amount of fuel being dumped over a narrow rev range. The development of high pressure direct injection systems that are electronically controlled has allowed for improvements all round. the engine does not need to be as rugged as it once was. The same time advances in turbochargers have led to a less on off affair with the turbo diesels power delivery. i dont think boost levels have changed that much have they? upto 1 bar/15psi

[wgknestrick]

2 stage compressor (turbo and piston) is more efficient than a single stage (piston) as your pressure ratios increase. You need to control the heat generated from compression and it is much easier to do in between stages, ie intercooling.

[KevinSalyer]

The compression ratio of diesel engine is the ratio of the total cylinder volume to the combustion chamber volume. The change of the compression ratio not only affects the power and economy of the diesel generator, but also affects its start-up performance. Each diesel engine is designed with an optimal compression ratio, but in use, the compression ratio is often reduced due to the influence of various related factors.
I The piston is low at the end of compression
II Combustor volume is larger

[NL_Fer]

Turbochargers have improved allot. From fixed geometry to a variable geometry, twin stage units. They start boosting even on idle, releasing the clutch. Also injection has improved, increased fuel pressure (2000 bar) and smaller droplet size.