Why is injection pressure for diesel engines so high?

Recently, in another report, I commented on the sky-high pressure inside the fuel rail of a modern 'common rail' diesel engine, which typically operates in the vicinity of 2000 atmospheres.

That's staggeringly high. Then I got the feedback, right:

So, let's break down why this is so.

Transcript

His royal Roman highness (above) is talking about the pressure in a modern diesel fuel rail. And yeah, I am sure about that. As sure as ithe’s a lazy nut - for failing to do even basic online research using Google and Wikipedia. Just another third-rate intellectually lazy loser.

2000 atmospheres, broadly, is correct. It’s huge. Some engines are up to 3000 atmospheres. So, essentially, here’s why, in case you’ve ever wondered:

3000rpm is 50 revolutions per second - 25 combustion events per second, per cylinder in any given four-stroke engine

3000rpm is 50 revolutions per second - 25 combustion events per second, per cylinder in any given four-stroke engine

OPERATING SPEED

If you think about a diesel engine at say 3000rpm. That’s 50 revolutions per second. Fifty! Two revs per cycle - each cycle comprising suck, squeeze, bang, blow. Like pornography, only hotter and a lot faster. The whole cycle takes one-25th of a second. 40 milliseconds. That’s for all four parts - sucking, squeezing, banging and blowing. (Redefines the term ‘quickie’.)

Also known as 'inlet', 'compression', 'ignition' and 'exhaust' respectively.

IGNITION TIMING

Let’s slow it all down. The window in time where all the valves close and the piston is on the way up. The squeeze is on. Compression. Piston gets near the top. It’s hot. It’s tight. It's ready to go off.

The fuel has a window of opportunity that is very brief - just a few milliseconds. You spray it in, it autoignites. Boom. Has to be exactly right.

Injection pulses - as many as five per combustion event - occur at very precise times in relation to the position of the crankshaft

Injection pulses - as many as five per combustion event - occur at very precise times in relation to the position of the crankshaft

INJECTION EVENTS

Except it’s not just one little spurt of fuel. It’s as many as five - two little spurts up front, a big one in the middle, and two little follow-throughs. The first two take out the mechanical knocking sound, main spurt delivers the bulk of mechanical work, and the latter spurts help clean up the emissions. A high-speed Caligula-esque orgy of spurting precision. 100 times a second, on a four-cylinder engine at 3000 rpm.

These injector events have very precise delivery schedules and volumes. And the windows of opportunity are millisecond-critical. It’s a beautifully synchronised ballet - like Swan Lake on crack - and it adapts in real time to all kinds of external influences - load, throttle position, speed changes, auto gearshifts - it’s friggin’ precise and a brain bender of complexity.

Modern piezoelectric injectors are incredible devices, but have no tolerance for impurities in the fuel

Modern piezoelectric injectors are incredible devices, but have no tolerance for impurities in the fuel

PIEZOELECTRIC INJECTORS

Injectors have these tiny holes - five to 20 holes per injector; about 150 microns each in diameter. And they open and shut manically and precisely thanks to the miracle of piezoelectric control. To make them work, you need a reservoir of fuel at super-high pressure. Literally busting at the seams to get in there.

You can think of the injector as a floodgate and the fuel rail as a dam with a pressure on the bottom of the dam wall at 2000 atmospheres. Or 200 megapascals. If you want to build a dam like that, with pressures like that, out here, the wall would have to be 20 kilometres high. (Like it would be if you were able to dive 20 kilometres deep into the ocean.)

So good luck with that.

The pressure in a direct injection fuel rail is magic - which is a good name for technology generally if you don’t understand how it works.

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