In a modern common-rail injection system, fuel is drawn from the tank via a conventional low pressure pump of the kind not dissimilar to petrol engines, then fed to a high pressure pump which produces absurdly high pressures in the order of 30-40,000 psi which is buffered within the common rail reservoir for onward distribution to the injectors. My question this time is, how is this extreme high pressure generated? What kind of pump can do this and how is it powered?
Common rail is actually quite an old idea but has only come into vogue recently due in part to emission requirements.
The pump is driven by the engine, the pump is often (always?) a cam driven sprung loaded piston with a non return valve after it, there may be an intermediate pump stage inside the pump to get the pressure up a bit before the serious pushing. The high pump pressures mean that tolerances are very tight and the slightest bit of crap can wreck the pump in seconds.
OK, so if I have this right.... The whining sound you hear when you turn the ignition on is the electrically powered low-power pump. The high pressure pump becomes active just by engine cranking alone? It's
*mechanically-driven* and provides this high pressure regardless of whether any other (electronic) system in the vehicle is functional or not?
as far as I know the hp pump is always engine driven. If there is no lift pump feeding it then it may not produce any pressure at all. It may also have an elctrically operated valve which would release pressure when no electric goes to it (that was how the stop solenoid works on older standard diesel pumps.
However, with no electrics the injectors will not open, so the engine cannot run.
I dunno, NP. It's not really my area, but I'm struggling to see how those kind of pressures could be achieved solely from the battery. You have to bear in mind that from a cold start, the battery is already doing a hell of a lot of work in a diesel-engined vehicle. Especially if it really is a cold start in every sense of the word at -15C at 6am. The cold cranking amperage plus whatever the glowplugs consume, plus whatever else is taking current at first switch-on, is going to leave precious little spare capacity for generating 1000 bar (or whatever it is) for the injectors. So for that reason alone I'm guessing (and it is only a guess) that the high pressure pump runs off a purely mechanical mechanism that derives its power directly from the engine's rotation.
The low pressure pump in often/always at the fuel tank under the rear seats. I've always wondered why on my car (a Peugeot 308) the pump *always* sounds for about 30 seconds after I fill up the tank and then start the engine, whereas it often doesn't sound if I start the engine without filing the tank. Could it be that a full tank amplifies the noise of the pump - ie that it *does* always come on as the engine is started, but I only *hear* it when the tank is full?
No - if no pressure is provided by the electric pump, it usually prevents the engine being cranked or stops the engine if it fails whilst being driven.
Mine has an in tank pump and a second pump under the bonnet. Neither can be heard unless you listen via the filler/ under the bonnet. They run for 30 seconds then time out if the engine is not cranked before the 30 seconds expires. I would epect a pump to be more audible, with a near empty tank.
This extremely high pressure injection pump is mechanically driven by the engine crankshaft output (it's literally part of the pumping losses common to pretty well *any* sort of heat engine whether it's the boiler feed in a steam engine or the induction and compression of the air impounded into the cylinder(s) of any ICE).
In this case, the amount of fuel being pumped per power stroke of each cylinder in a car engine, is just a few millilitres at most so although we may be looking at the extremely high pressures required to both crack open the injector valve and overcome the initial pressure from compression alone of the charge of air (several hundred PSI), swiftly followed by the several thousand PSI pressure raised by the process of combustion, the power (energy delivery rate) represented in the equation of pressure times volume of delivered fluid per minute is a mere fraction of a horsepower (certainly unlikely to exceed one percent of the power output delivery of the engine itself - I don't have any figures to hand, this is just an "engineer's guestimate").
The main distinction between a petrol engine and a diesel engine lies in the way the fuel and air is combusted. In a petrol engine (whether carburetted or fuel injected), the fuel and air are premixed to a narrowly defined optimum ratio before being compressed just prior to ignition, a process normally initiated by an electric spark, that results in rapid burning (not detonation!) of the compressed fuel air mix. Unfortunately, the difference in conditions between optimum efficiency and engine performance and engine destroying detonation conditions are so small that there is very little margin for error, so little in fact that modern microprocessor controlled engine management systems incorporate detonation sensors to sense when to 'detune' the engine away from the transition line between best possible performance and efficiency conditions and undesired engine destroying detonation conditions.
In a diesel engine, the required amount of fuel is injected separately into the already inducted and compressed charge of air in a precisely metered dose determined by the accelerator pedal input. The air charge in a diesel engine is so highly compressed that its temperature becomes raised well above the ignition temperature of the injected fuel which starts burning on contact with the compressed charge of air, heating the air volume which raises the pressure which drives the piston during the power stroke. The way the fuel burns in the cylinder of a diesel engine is not unlike the way a spray of fuel (eg white spirits) would burn when sprayed from a hand operated spray cleaner bottle past a handy source of ignition such as a lit zippo lighter held in the other hand an inch or so away from the spray nozzle (a trick we used to do now and again with the white spirit spray cleaning bottles in work - leaving behind an odour reminiscent of jet engine exhaust fumes).
This method of burning the fuel in a diesel engine cylinder immediately eliminates the 'explosive ignition' risk inherent in the petrol engine. The worst that can happen is over-fuelling, causing unwanted pollutants of incompletely combusted fuel/air products (black exhaust smoke) to be emitted which represents not only an environmental health hazard but also a waste of fuel and hence loss of engine efficiency.
Since the air intake of a diesel normally remains unrestricted throughout the whole of its power output range, it is able to translate much more of the fuel's heat energy input at tickover to mid power output settings than any similarly rated petrol engine is capable of achieving.
The less sophisticated spark ignited petrol engine has to burn more fuel to maintain tickover just to compensate for the throttled down air charge which needs to be heated to a higher temperature to maintain tickover speed compared to the case of the diesel which has a full charge of air to absorb the heat energy from the short pulse of injected fuel. Much more of the heat energy is translated into useful working pressure than in the case of the low density volume of air in a petrol engine running at tickover speed.
It's only when a naturally aspirated diesel engine is running at maximum power output that its efficiency drops down to little more than that of an equally power rated naturally aspirated petrol engine and, since a typical road car engine spends most of its time being operated at around
10 to 20 percent of its maximum rated power output, a diesel engined road car will return a much better fuel economy in spite of the extra weight penalty imposed by the design requirements of high compression operation.
The main advantage of a petrol engine over a diesel engine is its higher power to weight ratio and increased throttle responsiveness due to its 'Living on the edge of destruction' method of fuel combustion which permits higher useful revs per cc of swept cylinder volume (the bigger the volume, the longer it takes for the combustion process to complete).
The odometer can operate independently of the engine running, the camshaft cannot. The driven wheels cannot drive without the engine running and the drive comes from the crankshaft rotation.
It is true to say that trees and hence coal are derived from sunlight and hence are in the very long run 'renewable' energy sources.
Can you say which car engines have a common rail pump that is 'driven' by the camshaft?
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