Alfa Romeo Diesel Engine Bay

Alfa Romeo JTD Engine Bay

The Alfa Romeo JTD engine

Ten years ago a revolution started and it was labelled JTD; the diesel engine had come of age. I know some of the most fervent petrol heads amongst the Alfisti will still not believe it and will be turning the page at about this point in the article but please, please do read on. I didn't believe that a diesel could be any good until I drove one, and now I have a 1.9 JTDm and love it, but I do have some petrol Alfa's as well!

All JTD's are common rail diesels and they share a lot of design ideas with both the Twin spark and the JTS petrol engines. Rather than the old fashioned mechanical pump that both pressurises the fuel and times when the fuel is injected into the engine, the JTD has a high pressure pump that runs off the cam belt. This supplies fuel to a rail which is attached to all the injectors, hence the name "common rail diesel". The fuel in this rail is at massive pressures, around 21,000 psi as opposed to about 50 psi in a twin spark engine, so it is necessary to take extreme care when working with this system. From this "reservoir" the electronic fuel injectors, which work in a similar way to petrol injectors but are much more robust, squirt fuel directly into the combustion chamber in exactly the right amount at exactly the correct moment in time. The amount of diesel injected is calculated by a computer or "ECU" this takes data from the crank angle sensor, temperature sensor, air flow meter, fuel pressure sensor and air pressure sensor all of which are similar if not the same as the ones on petrol engines. From this data the ECU decides how much fuel to inject and when to inject it. You will have noticed by now that there are a lot of similarities between modern petrol and diesel engines and with the ever increasing compression ratios of petrol engines and direct injection on the JTS. I wonder how long it will be before car engines become multi fuel like some military vehicles are or at the least they will be able to take a much wider variety of fuel than they do at present. JTD engines are also turbo charged which helps to give the JTD its superb power output.

The JTD has evolved into the JTDm, where "m" stands for multi jet. While this is also a common rail diesel it runs at slightly higher injection pressures than the original JTD and also has four valves per cylinder which are operated by twin overhead camshafts, this aids volumetric efficiency, the second camshaft being gear driven off the end of the first. The JTDm's major advance however came with the injectors, these can inject 10 times faster than the JTD's injectors and this allows them to inject fuel multiple times per power stroke of the engine. This is used to increase the efficiency of the fuel burn by injecting the fuel in various strategies, at low speed or cruise the engine gives a small pilot injection followed by one large one this helps to keep the engine very quiet however on full throttle it just injects once. The injection strategy is worked out by the engine ECU and this is all programmed by the engineers at Alfa Romeo. The very latest version of the JTDm also has a very clever self cleaning soot filter in the exhaust system to make the emissions even less harmful. This will leave only the NOx in the exhaust gasses and engineers are working on reducing these by injecting urea into the exhaust which to those of you who know is an interesting concept.

The JTD engines respond very well to tuning as the turbo charger can be easily reprogrammed to keep forcing air into the engine. So unlike normally aspirated Alfa Romeo engines where it is quite difficult to get more air into the engine and therefore quite difficult to get more power from the engine, with the JTD it is simply a question of reprogramming the ECU to increase the boost pressure and get significant power gains although obviously if you use this power your fuel consumption goes down. I can still feel those petrol heads amongst you who have bothered to read this far saying "its all low down power" and you are correct with earlier JTD engines however with the 2.4 JTDm in the Brera and the 159 this is significantly changed and while it has masses of torque as the revs rise this transforms nicely into power; they are well worth trying.

Having sung the JTD's praises it is probably best to give you a quick tour of the engine bay (see image):-

1. Fuel injectors, these have multiple injection holes round the nose of the injector and they use electro magnets to open and close the injection nozzle, they are controlled by the engine ECU which is in the passenger foot well.

2. Dip stick, used to check the oil level to be used REGULARLY on all Alfa engines.

3. Vacuum pump, Diesel engines don't have a throttle butterfly in the conventional sense and the inlet tract could theoretically be open to the air at idle so no vacuum would be produced, a vacuum is needed for the servo assistance on the brakes and to activate the turbo.

4. Fuel filter makes sure there are no particles getting through to the injectors and also separates out any water that might get into the system. There is also a lift pump in the fuel tank that primes the fuel filter

5. Throttle butterfly, having stated in 3 that diesels don’t have throttle butterflies here we have one but it isn’t here as a throttle. It is a flap that works in conjunction with the exhaust gas recirculation (EGR) valve to "seal" the engine on over run so no new air is pumped through the engine thus reducing pollutants. Those of you with diesels especially m jets may have noticed a slight "pop" when you turn your engine off that is this valve closing.

6. Exhaust gas recirculation valve.

7. Pipe from intercooler, this pipe goes down to the intercooler in the inner wing which reduces the temperature of the air going into the engine making it more dense so you can get more air into the engine and increase its power output.

8. Turbo charger, somewhere down here is the turbo charger

9. Boost pressure sensor, this measures the temperature and pressure of the air going into the engine.

10.Cam belt


Related Links: JTD Cam Belt Change Guide | |

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