operating coolant temperature?

[Kremmen]

I suppose another consideration in how you dress for driving.

I'm not one to drive in any outside coat, gloves or hat. A lot do and probably similar to wearing full stage 3 kit indoors.

[embee]

It depends where the coolant temp is being measured. I'm guessing the system uses a "thermostat on inlet" arrangement where the stat regulates the flow of low temp coolant from the rad into the pump/engine circuit rather than regulating the flow of hot coolant out to the rad.

The "traditional" arrangement of "thermostat on outlet" from the engine diverts hot coolant from the bypass circuit out to the radiator once it reaches the stat opening temp. The problem/consequence of this is that the engine gets fed a slug of cold coolant from the rad going through it until it gets round to the outlet again and the stat reacts and starts to reduce the flow to the rad. This inevitably results in hot/cold cycling which is undesirable. A stat on outlet will usually be set at around 88C typically.

A stat on inlet gets the full bypass flow over it, going back to the pump inlet, and when it reaches typically 80C it starts to let cold in from the rad, that immediately meets the bypass flow and mixes over the stat itself so it will close down enough to keep the engine/pump inlet temp pretty much steady at about 80C. The hot/cold cycling is greatly reduced.

At high load you expect/engineer the cooling system to give a temp rise (deltaT) across the engine (or temp drop across the rad if you prefer) of something like 7 to 10degC, so an inlet temp of about 80-82C is pretty normal for a stat on inlet system. At light load the heat input into coolant is low, so the deltaT is small anyway, maybe only a couple of degC.

Regardless of where in the system the temp is monitored, at low load/idle the deltaT is small, in which case 80C is more or less what you'd expect under typical circumstances anywhere in the engine coolant circuit. Fan switches/control temp would normally be at engine outlet (or possibly the rad header tank) to come on when the coolant temp exceeds a set value, by which time the stat will be fully open anyway to give full rad flow and closed bypass.

[ColinB]

... its more efficient to use  the heated seats  initially.    ... heated seats are a more efficient source of heating.

Forgive me for veering slightly off topic (and apologies for paraphrasing a lengthy and well-informed observation) but I’ve seen this kind of comment before and it puzzles me. I want heat in the cabin as early as possible not to warm me up, but to demist the windscreen so I can actually drive somewhere safely. I hate wiping the inside because that always results in a streaky mess, so I sit there for a few minutes until the engine produces enough waste heat to clear the screen. If you rely on the heated seats and steering wheel, how do you get the screen clear?

[Marco1979]

Getting the wind screen clear of condensation is the most important thing. Heated seats won’t help. So you need the engine to be a bit warmed up and heat the screen and indoor air.

I must say: my Jazz stays pretty dry so even with a lot of frosting on the outside (removed manually) there is hardly any condensation on the inside. But I do drive longer distances (30 to 50 miles mostly).

[Jazzik]

If you rely on the heated seats and steering wheel, how do you get the screen clear?

I don't get the windscreen clear. I KEEP it clean by parking in our garage...

[Mark Dirac]

It depends...

Thanks for that embee - most insightful.

Could you suggest why my thermostat is sending hot water to the rad (or accepting cool water from the rad) when there's still work to be done to heat the cabin, and even worse why it is starting the engine instead of simply diverting hot or warm water from the radiator?

[BROC]

Thanks for answering my question Zaier. And thanks to Kremmen for replying to pete-preston.

So, if my thermostat is opening at the same temperature as others, then I reckon there's a design flaw. Because...

My Jazz, just like many here, starts its engine just to provide cabin heat. So what? Well, my ODBII scanner reveals that this is happening even with the thermostat open. That's crazy. Instead of starting the engine to produce heat, it would be more sensible to close the thermostat.

I can drive 20 miles (country roads) with the coolant at operating temperature, at 78 degC, and yet the engine will still start when I start the climate control (AC off), and stop/EV mode when I turn the CC off. So the cabin heater is demanding that the engine starts up to generate heat even though the coolant is hot and indeed is being sent to the radiator to be cooled down.

It's as though the design is such that cabin heating demands and radiator cooling demands are not co-ordinated.

Does the engine not need to run to circulate the engine coolant? I am guessing the engine water pump will be driven by the engine

[Jocko]

Does the engine not need to run to circulate the engine coolant? I am guessing the engine water pump will be driven by the engine

Modern systems need the water pump running to circulate the hot water through the engine and to the heater. In the old days of tall radiators, they worked by thermosyphon.

[embee]

Does the engine not need to run to circulate the engine coolant? I am guessing the engine water pump will be driven by the engine

It's pretty common these days for cars to use electric water pumps. Some use bigger pumps for the whole engine cooling system, a lot more use a small electric pump for the cabin heater (and sometimes for cooling turbo bearing housings after shut-down to reduce oil coking etc).
I would have guessed the hybrid Jazz would have an electric pump for the cabin heater, but it might rely on a mechanical pump if the possible engine operating conditions are relatively few.
Does anyone know if the Jazz has an electric pump or pumps?

Back in the day many of the engines I worked on had oversized coolant pumps simply to provide sufficient cabin heater flow at idle, it was a pretty inefficient way to do things.

[embee]

Could you suggest why my thermostat is sending hot water to the rad (or accepting cool water from the rad) when there's still work to be done to heat the cabin, and even worse why it is starting the engine instead of simply diverting hot or warm water from the radiator?

I imagine it is a compromise startegy, you have to choose some deciding factor as to whether the engine runs or not. Cabin temp might not be a first priority compared to engine system temp, they probably don't want the engine doing many short term stop/start cycles (probably annoying for the occupants apart from anything else), so some considerable hysteresis might be programmed in, letting the engine run for longer than perhaps absolutely necessary. I don't know, just guessing.
The engine stat will regulate the temp the coolant reaches in the engine circuit through warm-up or at light load. Once the temp reaches 80C or whatever it will hold it there, however the cabin might not have reached the demand temp yet but will still get 80C coolant sent to the heater.
It's often the coolant flow rate rather than the inlet temp which  dominates heater performance, if the flow rate is small the temp drop across the heater matrix can be quite big. If say it drops from 80C to 40C the average is 60C. If you double the flow rate the drop might be from 80C down to 60C, so the avearge is 70C . If air is at 10C and the coolant ave is 60C the difference is 50degC, if the ave coolant temp is 70C the difference is 60C so the heat transfer is 20% greater, that's why heater flow rate is important (example numbers just to give an idea ....)

[Jocko]

The Jazz GR has an electric water pump.

https://www.autopartspro.co.uk/kamoka-19171513

[embee]

That looks like a variable output mechanical pump, a quite popular way to optimise temperature control while minimising parasitic energy. Reducing flow when it isn't required for heat removal will reduce the losses from the engine, improving fuel economy. Reducing flow during low load warm-up can speed up the temperature rise within the engine, reducing friction and improving thermal efficiency etc.
They usually have movable vanes or baffles which allow internal recirculation within the pump, there is typically an electrical actuator and position sensor to control it.

[Zaier]

Jazz engine has an electric water pump in every version, on the teardown available on "Marklines" it is clearly visible.
That's why the heather can also work when the engine is not running over certain temperatures.

[Lord Voltermore]

I find the defrost button works surprisingly  quickly. I did wonder if it actually used an electric hot air  heating element which I believes is what full EV's have to do , with a significant range penalty in winter.      But I think it  just concentrates whatever heat comes from the engine as soon as its available, rather than trying to heat the whole cabin.  An electric water pump probably helps here. 

Which is why heated  seats are not a decadent , unnecessary, , luxury in Hybrids.    They quickly get you snug and warm while engine heat is concentrated on the windows.  An extra that I would certainly miss.   Heated steering wheel also nice but less essential.

The wing mirrors defrost very fast. It would be nice to have the windscreen defrost that quick,.The technology exists but would add a lot to the price.

[John Ratsey]

I find the heated seats useful in cold weather as, unless there's a need to demist the windscreen (for which a chamois leather pad is the first line of defence), the heated seats provide some comfort while leaving the engine to get warm and more efficient. This is particularly relevant in urban journeys where supplying heat keeps the engine running much more (even when the vehicle isn't moving) than during warmer weather. Yes, the seats use some electricity but that's not a problem when the engine is running more while trying to get itself warm. Once the engine is warmed up (< 5 miles in cold weather) or on the open road then the heated seats get turned off.